Author Topic: stripping paint  (Read 4310 times)

jjb-w116-hu

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stripping paint
« on: 19 January 2011, 07:12 PM »
hi guys, not sure if my previous post got lost some where re this but am looking for some wisdom - what is the concensus on stripping paint? i have heard many say that media blasting is best, provided its done correctly and others say you can acid strip panels, hoods, trunk lids, doors etc - but then i have heard issues with acid stripping re removing some silicon?
i have been interested in acid stripping my doors only as there seems to be so much built up crud once you remove the inner panels and body mouldings...

appreciate any tips guys ,

cheers James

thysonsacclaim

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Re: stripping paint
« Reply #1 on: 19 January 2011, 07:50 PM »
Hi James,

Regarding stripping, I can only talk about the chemical methods of doing so. I'm not very familiar with silica / sand blasting, so I'll let someone else answer that.

You didn't mention having a shop do it, so I'm assuming you are wanting to do this yourself, so this is fairly long, detailed reply.

As far as acid stripping goes, most rubbers, plastics and polymers are considered to be mostly chemically inert. That is to say, they don't react with most other chemicals very easily. In fact, many of those chemicals I mentioned, such as plastic, are actually more inert than glass. Hydrochloric acid, especially, does not react with hydrocarbons like petrol, tar, oil, etc. Since most plastics, rubbers and polymers are hydrocarbon based... you get the idea... it won't generally react.

Labs use plastic containers with rubber, plastic or silicone seals for highly corrosive chemicals, such as hydrofluoric acid, which can dissolve glass. There are exceptions, but as a general rule silicone is not dissolved by hydrochloric acid. Hydrochloric acid is also the only acid you would want to use for this. Do not use sulfuric or nitric acid.

You may hear it called muriatic acid, this is the same as hydrochloric. Muriatic acid usually comes between 30-40% hydrochloric acid mixed with water. That is more than enough; anything stronger is much to dangerous to use for what you need unless you dilute it. I use hydrochloric acid quite a lot (weekly and sometimes daily) for paint stripping, for cleaning dirty aquariums (which were sealed with silicone), in the lab, and for removing chrome plating. It's quite useful.  :D

To be safe, my advice to you would be to buy a small container muriatic acid and do a spot test on the silicone/rubber. Pour a small amount of the acid into a glass container and use a small dropper and apply it to the silicone. Let it sit for up to 24 hours and check on it again for any degradation of the silicone. It is unlikely, but the 'silicone' may be another similar sealant which looks like silicone, but is not as inert as silicone. It is best to test before you start.

You may want to try diluting the acid because you may be OK with using just 15%-20% acid. I would test this by diluting it 50/50 and see if it works just as well or nearly as well as the regular concentration. If it does, your acid will last twice as long and save you some money, which is always a plus  ;D

There is one thing you need to know if you are going to dilute this yourself: Do NOT add water to acid. You always add acid to water. In other words, if you wanted to mix 500mL of water and 500mL of acid (1:1 ratio), you would pour the acid into the water. If you do the reverse and pour water into acid, you risk having it instantly boil up and splash up into your face, hands etc. Just think "Add Acid." Make sure you use a glass or plastic container when mixing. You should use, at a minimum, rubber gloves (dish gloves should be fine) and splash resistant goggles and work outdoors. It won't instantly melt your skin off or anything, but you don't want to get it in your eyes, nose or leave it on the skin very long without cleansing with water. It would be wise to use a mask for your nose and mouth, similar to what you use when painting. It may seem like overkill, but it can cause quite a lot of damage. :o

Here is the Material Safety Data Sheet for hydrochloric acid so you know a bit about it and how to handle it: http://www.jtbaker.com/msds/englishhtml/h3880.htm


If you do choose to strip with chemicals, just do not use acetone. Acetone does absolutely dissolve silicone along with most other plastics and rubbers.



Cheers and good luck,


Adam
« Last Edit: 19 January 2011, 08:04 PM by thysonsacclaim »
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jjb-w116-hu

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Re: stripping paint
« Reply #2 on: 19 January 2011, 08:10 PM »
wow thanks for that detailed explanation adam, this will help me with the smaller areas and bits but i might considering all that just let a strip/dip shop dunk my doors in a bath. i'd obviously have to strip the doors of everything but the more i go the more i think i dont want to drop the kind of gold on a nice paint job if the underlaying work is half cocked or not as nice as it could be.

cheers again! :)

thysonsacclaim

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Re: stripping paint
« Reply #3 on: 19 January 2011, 08:21 PM »
It can be very labor intensive to do it on your own. I usually only acid strip on a small scale simply because it requires so much acid to do it on a large item. The largest area I've done is a chrome bumper and some rims. The other plus of having a shop do it is that they'd likely be liable for any damages.

A full dip in acid is the best way to go about it, or to at least let it pool on the surface. Shops obviously have access to much more acid than the average person and they also are usually able to recycle it for use again. They also sometimes pair it with electrolysis, which can give a very outstanding result.

On the flip side of that, it is very satisfying to do your own work sometimes and you can also buy the proper acid in bulk as long as it is legal to do so where you live (it is sometimes used for manufacturing illicit drugs, so I know there are quantity limits in some areas of the world). Here is an example: http://www.dudadiesel.com/search.php?query=hydrochloric&affiliate_pro_tracking_id=17:32:US 5 US Gallons is roughly $40.

good luck again with your project, whatever route you go I hope it turns out nice  8)
« Last Edit: 19 January 2011, 08:24 PM by thysonsacclaim »
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craigb

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Re: stripping paint
« Reply #4 on: 21 January 2011, 09:26 PM »
I have good results with off the shelf paint stripper from the hardware store. My vintage car club had a morning with a trade school lecturer and this topic came up. If the internals of your doors are not rusted or damaged, undersides of bonnets etc., you may as well leave the original factory paint intact since this has been bonded when new, lasted all this time so you are not going to do better. If you are repainting faces of panels and not doing frames etc, then leave a cm or so at the edges, can mask this but you don't want to disturb that edge if you don't need to. After the bulk of the paint is off you sand the edge for a good key but be careful not to go through to the metal. As for the stripping itself, give it a course sand first to help the chemical penetrate, then paint it on and get some old plastic to just lay over the top. This will stop it evaporating and give maximum effect, Then use a scraper but make sure you clean up the corners with a file so it doesn't dig in. Wash off with lots of water.

Then you can do all your prep for painting. If the paint work on the whole is not damaged, then you can give a good sand, prime etc without going back to metal, but if there is any crazing or other faults then bare metal is the only way to go.

I have heard people rave about soda blasting I think they call it but cant vouch for it. Never had any problems with the other cheap  methods.

but see what others say and let us know if you have a good experience with something.
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thysonsacclaim

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Re: stripping paint
« Reply #5 on: 22 January 2011, 01:36 AM »
I heard somewhere--and I do not know if this is true--that Mercedes used quite a few coats of paint on their cars which makes them very resilient to wear and rust. The only evidence I have of this, however, is word of mouth from local 116 owners and the fact that every older Merc I have seen with factory original paint was pretty clean (similar to how well the interiors hold up). My car has had at least 4 owners... the DMV records don't show more than that... and the body and interior are in realitvely good shape. Mercedes quality at its best!  8)

I have seen domestic (USA built) cars 10 years younger than mine with far more rust problems. Whether this difference is because of the quality of the Mercedes paint, or the fact that the original Merc owners of the cars took care of them, I cannot say. I personally think it is probably a combination of both.

I second the idea of spot stripping the paint versus dunking the whole lot, as long as the paint is still in good shape inside. It may just need a touch up, too. This will make it easier for you, as it will take less time and since you CAN use acetone (aka regular paint stripper) as long as it isn't coming into contact with plastics or rubbers. Regular paint stripper works very well in my experience and is easier to handle than acid by a long shot. Acid, I think, is personally more suited to larger, more corroded items which require a full dunking and which may be made of materials that acetone will eat away or soften. Acetone is my solvent of choice, but it requires a bit of care to make sure you don't dissolve something important (gaskets, seals, etc).

The plus for mechanical (aka blasting) versus chemical (acetone/acid) stripping, which I didn't think of earlier, is that you'll get a course surface for paint adherence while the paint is being removed, so the metal won't need as much additional sanding after the blast. This is because it is usually etched very slightly by the blasting process itself. If you use chemicals, you will need to apply a little more elbow grease after removing the paint, since the chemicals will not etch or roughen the metal on their own.

One tip for you: if you do go with acetone, again try to buy in bulk locally, online or from a boating store. Marine paint stripper is usually acetone (check to be sure; may be different there) and is often sold in larger quantities since boats tend to be bigger than cars  :P If you're doing a lot of work, it will save you a bunch of money to do it this way.
« Last Edit: 22 January 2011, 01:39 AM by thysonsacclaim »
Adam


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