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BFS - Brute Force Stripping, Aggressive Chemical Sanding to remove Discoloration

Started by TelePlay, May 24, 2018, 05:43:14 PM

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TelePlay

Just finished taking all of the salmon discoloration off of an ABS pink 500. When doing that, I discovered a major Oh No! due to the stripping solvent (acetone mixed 50/50 with denatured alcohol).

The solvent mixture was quickly removing the salmon colored plastic but was leaving white streaks in the pink plastic after the acetone evaporated. It seems to be the case that while the acetone was dissolving the discolored plastic, it did so in two different ways. The dark, hard salmon colored plastic was softened and removed, it came off in rolling beads just like certain types of dirt rolls off a surface. But, the acetone was also putting into solution a very light to white colored plastic used to make pink which was left on and attached to the plastic surface when the acetone evaporated - in a few words, the stripping process was creating a "white" plastic paint which stuck to the surface quite well and became part of the pink, plastic housing. No matter how the solvent was used, it left permanent white streaks, a major Oh, No! at that point.

I got frustrated because nothing would remove the whitish streaks except acetone which just created more white streaks when it dried. Weaker solvents did not remove the white streaks, they were part of the plastic surface.

To resolve this new Oh, N0!, I developed a technique I call BFS, Brute Force Stripping. This starts with using the acetone mixture remove all discoloration from the housing, handset and caps ignoring white streaks and chemical sanding marks.

At that point, I had some really ugly looking plastics on my bench. Next step, using a 400 grit sanding block to wet sand all stripped plastics (about a half hour of total sanding time) until the pink streaks and sanding marks were gone. This left very nice, completely smooth but very dull original pink plastic components.

The next steps included the use of a 10% acetone, 10% Methyl Ethyl Ketone and 80% Denatured Alcohol mixture. this mixture was used with a soft cotton cloth swab (backed by a cotton ball) to remove high sanding marks between sand paper grits. Wiped one side with the solvent wet swab to remove the 400 grit marks and slightly soften the plastic so the next higher grit (600) would cut better.

Repeated the "wipe the surface" and "next higher" sandpaper grit to move up from 600 to 800 to 1000 and finally 2000. A sanding block was used on the flat areas to maintain a smooth surface (no sanding depressions). Loose sandpaper was used in the corners and wherever the sanding block would not work.

Next step was to mix up a buffing/polishing mixture. This was a 50/50 mixture of Novus 2 and the 10/10/80 solvent mixture. About 2 ml of each was put into a small. plastic medicine dispensing cup and mixed thoroughly. Just enough was mixed for the project at hand and any excess was discarded.

After the 2000 grip sandpaper, the solvent swab was use to wipe the surface, to remove the high sanding marks left by the 2000 grit paper and the Novus/solvent mixture was immediately applied to the solvent damp surface using a finger tip, spreading the Novus/solvent mixture evenly over the plastic. The Novus/solvent mixture was immediately removed using a Ryobi orbital buffer with a cotton bonnet.

The plastic after one buffing with the Novus/solvent mixture was mirror smooth, shiny and back to the original pink color.

==============

The quality of the final shine depends on the quality of the last sanding steps. Using 1000 and then 2000 grit on a sanding block, where one can be used, goes a long way to produce a mirror smooth surface ready to polish to a mirror shine.

Novus could make the mixture but they can't. If they put these volatile solvents into their mixture, they would evaporate off in the container head space and the acetone and/or MEK might weaken the plastic bottles over time on the shelf. By adding the solvent to the Novus 2 just before use, it creates a polishing mixture that soften up the plastic and lets the Novus 2 polishing agents work faster and better than just straight Novus 2 on a dry, hard plastic surface.

As I've said before, chemical sanding works but it is a seat of the pants process since no two plastics are the same (colored pellet mixtures differ from phone to phone and from manufacturer to manufacturer) but the general technique works, works well and works quickly to restore the original plastic color. The best part is, the more times one uses chemical sanding to restore color, the better they get at using the technique, the less fear they have of surprise Oh No's! and the faster a complete phone can be restored.

Bottom line is that there is nothing that can go wrong, within reason, that can't be fixed with a patch of wet 400 grit sandpaper and the use of a 10/10/80 solvent mixture (for ABS) to quickly get from 400 grit to the final polishing step and a mirror finish.

As long as care is taken before starting to determine the best solvent mixture to chemically sand or strip a certain phone (from pure acetone to a mixture of acetone with denatured alcohol for ABS and from pure denatured alcohol to a mixture of denatured alcohol with water for soft plastic), the major, major Oh No of destroying a plastic part can be avoided. Either plastic mixture is determined by starting with the weakest mixture, say 50%/50%, to see how it works on the plastic. If that does not work, try 60/40, or 70/30, 80/20, 90/10 and finally pure acetone or denatured alcohol for ABS and soft plastic respectively.

Too strong of a solvent will weld the fabric being use into the plastic, a major Oh No easily avoided by careful preparation. The point is to make a stripping mixture that allows a fairly rapid but controlled removal of the discolored plastic. And, keep in mind, the discolored plastic is harder than the original plastic so when starting, the solvent soaked cloth moves smoothly over the hard plastic but as it is removed, the original plastic will be softer and will "grab" at the solvent soaked cloth - a good indicator that the discolored plastic has been removed.

The best solvent mixture is one that relatively quickly removes the hard, discolored plastic but does not end up melting the cloth being used into the original colored plastic right below the discoloration. It's not as scary as it sounds. Best way is to start with a dilute solvent solution and work up to that which works best for the phone being restored.

Yesterday eats you up, it eats everyone up . . .

HarrySmith

Thanks for sharing that process. Quite a difference in that phone! Looks great! The first plastic phone I restored was a WE 500 that looked just like yours. I spent many hours sanding that phone! I learned from it though, I started with dry sanding and learned about wet sanding. I joined ATCA & TCI looking for info on restoring the color. I also learned I hate sanding! I have not sanded a phone since.
Harry Smith
ATCA 4434
TCI

"There is no try,
there is only
do or do not"

Key2871

The housing looks great, but your using some serious chemicals there. I hope good ventilation and seriously good gloves are being worn. For your sake, wow. One spark and your done for a while.
I had a ivory painted white call director once, and the paint was chipping off. Exposing a perfect white housing. After a few hours of wet sanding using 1000 to 8000 grit, then buffing, it came out perfect. I had tried chemical stripping before, but it attacked the plastic too much. So I went the other way.
KEN

Butch Harlow

You are a wild, wild man. MEK is crazy stuff. I did a little fiberglass scenery work years ago. If you use too much MEK it bursts into flames, clear flames. That housing looks fantastic however. I can't wait to try this on a donor phone sometime soon. I also somewhat despise plain old wet sanding, so this all seems like a route to go, especially on the lighter color ABS.
Butch Harlow

TelePlay

Quote from: Butch Harlow on May 24, 2018, 09:22:20 PM
You are a wild, wild man. MEK is crazy stuff. I did a little fiberglass scenery work years ago. If you use too much MEK it bursts into flames, clear flames. That housing looks fantastic however. I can't wait to try this on a donor phone sometime soon.

Yes, wild is what it is. And, yes, I've worked with fiberglass and while I enjoy the smell, I realize they are not good for the body and they are flammable organic compounds being given off. There is a huge gap between the material being used working with fiberglass and the minute volume of solvents used for chemical sanding.

I mix up the 10/10/80 ABS final solvent in a rubber corked 20 ml glass bottle (2/3 of an ounce). Easy to mix, 2 ml of acetone, 2 ml of MEK, fill the bottle with denatured alcohol and shake to mix. That volume is enough to do a complete 500 housing because a very small amount is used between sandpaper grits to speed up the sanding process.

As for the chemistry of the 3 solvents, they are all organic "fuels" (will oxidize, burn, easily using oxygen in the air if lit on fire). The flash point is the temperature at which the pure liquid is warm enough to evaporate enough of the solvent, turns into the gaseous form of the solvent, to light the fumes with a match and have it burn, not explode. Acetone has the lowest flash point, -4° F, MEK is next at 16° F and denatured is highest at 55° F. So, acetone is the most flammable of the 3. The human nose is quite sensitive to odors and my rule of thumb is if I can't smell it, it won't hurt me (with respect to being a fire hazard).

An important factor is the percentage of solvent in the air that can lead to an explosion. As with anything that oxidizes rapidly, burns, it must mix with oxygen from somewhere. The atmosphere contain about 20% oxygen (79% nitrogen and 1% everything else). Given the 20% of oxygen in air, somebody determined the amount of each solvent that would need to be mixed in air to create an explosive situation (the point at which the organic fuel has the exact amount of oxygen around it to completely burn in an instant given an ignition source - an explosion).

For MEK, it has to reach between 1.8 and 11.5% by volume in room air to be an explosive mixture, for acetone, the concentration if room air is between 2.5% and 12.8% acetone and for denatured alcohol, between 3.3% and 19.0%. If the concentration of any of these was more than 1% in the air, it would be quite strong to the nose and eyes so if I can't smell it, I am safe.

This also applies to solids such as dust in granaries which can explode from a spark source when mixed with just the right amount of oxygen. A video demonstrating a grain dust explosion is on YouTube

     https://youtu.be/bEShlfxB1Ik?t=2m45s

Same applies to other fuels including coal which these days is ground into a powder and mixed with pure oxygen to get a very complete and very hot "controlled explosive" flame in large boilers. Anything that burns, such as flour and wood dust can be made to explode when mixed perfectly with oxygen in the air. Unfortunately, these solvents evaporate and mix with room air and if the concentration of the solvent in the air gets into a certain range, a spark can set off an explosion. Natural gas is another fuel which if leaking slowly into a room will at some point reach an explosive mixture with the room air and an spark or open flame will set it off. Fortunately, the amount of solvents being use for chemical stripping is so small they will never get to an explosive percentage in room air.

Now, tipping the 20 ml bottle over near a flame is not good but I don't work with a flame nearby and I try not to tip over uncapped solvent bottles. But even if I did, if the 20 ml instantly evaporated into a 3' by 3' by 3' area over my work bench, that would produce a concentration by volume of 0.0015%, well below the explosive concentrations stated above. 

When chemical stripping, I use 1" squares of high bite, synthetic cloth backed by a small cotton ball. This eliminates ending up with a large cloth soaked with solvent. The small swabs use a very small amount of solvent so a little solvent goes a long way and the small amount used reduces solvent fumes evaporating in the work area. I rarely get a whiff of the solvents when working on a housing. I do use finger cots on 3 fingers of my working hand to keep the chemicals out of my skin. I've never used a fan to clear the fumes because a fan would dry the plastic surface too fast making chemical sanding/stripping difficult to do well.



Quote from: Butch Harlow on May 24, 2018, 09:22:20 PM
I also somewhat despise plain old wet sanding, so this all seems like a route to go, especially on the lighter color ABS.

I never liked wet sanding either, took way too long to get the discolored plastic off and always ended up with grit marks making me go back and try again, and again and again. The use of the 10/10/80 solvent between wet sandpaper grits speeds up the sanding process and does not require that sanding be done in one direction for one grit and the other direction for the next grit. I can move from 400 grit to polished shiny on one side of a housing in less than 10 minutes. Try it on a junker housing, just one flat area. Sand it with dry 600 grit paper to give you a rough finish and then use the wet paper with the solvent between grits to quickly work up from 600 to 2000 grit and the final buffing with a 50/50 mixture of Novus 2 and the final solvent to see how it works. Every time I do this technique, I am always amazed at how well it ends up after just one buffing with the Novus/solvent mixture.

It the same process for soft plastic or ABS, only the solvents are different. Acetone for ABS and denatured alcohol for soft plastic. I rarely use full strength solvent for removing the discoloration. 75% acetone and 25% denatured alcohol is usually good for ABS stripping/sanding and 75% denatured alcohol and 25% distilled water is usually good for soft plastic. Each plastic is different so the goal is to find the solvent mixture that get the discoloration off without being too aggressive, taking too much off while stripping.

When practicing on a junker so if the first try does not turn out well, sand it down with 600 grit and start over. Repeat until you feel comfortable doing this before moving on to a collectable phone.



Yesterday eats you up, it eats everyone up . . .

Butch Harlow

Thanks Dr.John, plastic wizard. I really appreciate the in depth explanation.  This is all very interesting indeed. I assume I will have to move my lab onto the balcony. The wife might not appreciate me mixing chemicals on the dining room table.  Don't ask me how I know methyl ethyl ketone burns, but it involves the GM design center and a tight time limit, and trying to work too fast at 3am. I can't wait to test this all out. Excellent info, it seems the way to go. It will save my achy old hands a lot of trouble. Thanks again!
Butch Harlow

FABphones

I came across this thread and thought I would try out the Novus/solvent mixture mentioned here.

This handset had been snapped into two pieces by a former owner. After removing the old glue and performing an 'Acetone melt' repair I thought I would see how much of a shine I could get. Am very pleased to have found this mix. Thanks for posting the detailed info.

Photos show the dull handset before repair, and after Novus/solvent polish:
A collector of  'Monochrome Phones with Sepia Tones'   ...and a Duck!
***********
Vintage Phones - 10% man made, 90% Tribble
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Doug Rose

Kidphone

SUnset2

For what it's worth, a company I used to work for used 100% MEK to solvent weld ABS, so I'm assuming that's the most aggressive solvent.  Of course, it was used under a fume hood, as it is highly toxic.  For Tenite, acetone would likely be more appropriate.