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How Much Finish Do You Have To Remove To Repaint A Solid Color?


fyb
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Hey all! I want to refinish my MIM Strat body and it's got a polyester finish I believe. I want to repaint it a soild color, so I'm not going to bother stripping down to wood. How much finish do I need to remove before I spray primer?

Should I take off the clear coat or can I just scuff sand?

Thanks for the info!

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If an existing poly finish is pretty well intact, I'll usually just do any necessary drop fills with cyanoacrylate, and level those. Then I sand the finish with 320x until there's an even scratch pattern and start spraying primer. Sometimes it's tricky to get finishes to stick to a poly undercoat, but if you use something like KILZ (which sticks to just about anything) for the first primer coat you'll do OK.

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Sure. I couldn't hurt. Depending on what sort of finish I plan to spray over the primer coat, I'll sometimes just use plain old shellac (which definitely sticks to anything) to give the primer -- and to a lesser extent the color coats -- something to "bite" into.

The problems crop up when someone just scuff sands the poly and then starts hosing on some finish out of a poof can ... which usually pops right off during the sanding/buffing process(es) like a flea off a hot skillet. Since most poly finishes are virtually immune to solvents, any solvent-based finished sprayed over them won't get any "bite" into the underlying surface, and ends up flaking right off.

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The problems crop up when someone just scuff sands the poly and then starts hosing on some finish out of a poof can ... which usually pops right off during the sanding/buffing process(es) like a flea off a hot skillet. Since most poly finishes are virtually immune to solvents, any solvent-based finished sprayed over them won't get any "bite" into the underlying surface, and ends up flaking right off.

I have a few problems with this statement.

1. Scuff sanding is normal, and quite required, for joining dissimilar finishes to each other, it is giving you a physical adhesion for your new finish to bite into as opposed to relying on a solvent to soften up the underlying finish, as in a film finish.

2. Every finish has a solvent that will cut or thin it, even poly, every finish that I know of is a solvent-based finish.

If I had to rewrite what you wrote to be correct, it would go something like this:

"The problem crops up when someone just steel wools the poly and then starts hosing on some sort of film finish, which can then separate itself and pop off due to there being no solvent adhesion or physical adhesion taking place, there is nothing bonding the two dissimilar finishes together properly. Since most poly finishes are virtually immune to solvents resoftening them once they're dry, any typical solvent-based finish sprayed over them won't get any "bite" into the underlying surface, and ends up flaking right off unless you scratch sand the poly thoroughly to form a physical adhesion between the two dissimilar finishes since you can't rely on the solvent softening up the underlying poly to get your chemical adhesion.

There is chemical adhesion, and physical adhesion.

Chemical adhesion comes from the solvent used forming a bond between old coat and new coat by softening up the old coat, as in film finishes.

Physical adhesion comes from scratching the old finish with sandpaper to form a physical adhesion since there is no chemical adhesion to rely on. :D

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Ouch. Can I come out from under the bed now?

Note that I said MOST (not ALL) poly finishes, and VIRTUALLY immune (not UNIVERSALLY). One of the only solvents I know of that effectively softens/etches a cured polyurethane or polyesther finish is methyline chloride, and I'm unaware of any commonly-available finish that contains that particularly nasty solvent. My point was/is that it isn't a good idea to rely solely on chemical adhesion in these cases.

Therefore, IMHO (with 30+ years of guitar repair and building experience behind me) the primary factor in obtaining adhesion onto a cured poly finish is the mechanical -- or physical if you prefer -- adhesion from sanding the surface. Obviously we could go round and round over this, but I'm done. Believe me, I'm done.

Te futueo et equum tuum.

Edited by guitarzan
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Wow, you are -really- thin skinned if you took that much offense to a simple corrective post. :D

People are here to learn about these things, and it's important you get your terms and your explanations right, else no one will learn anything of value from your post, no big deal beyond that. :D

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Sure. I couldn't hurt. Depending on what sort of finish I plan to spray over the primer coat, I'll sometimes just use plain old shellac (which definitely sticks to anything) to give the primer -- and to a lesser extent the color coats -- something to "bite" into.

The problems crop up when someone just scuff sands the poly and then starts hosing on some finish out of a poof can ... which usually pops right off during the sanding/buffing process(es) like a flea off a hot skillet. Since most poly finishes are virtually immune to solvents, any solvent-based finished sprayed over them won't get any "bite" into the underlying surface, and ends up flaking right off.

Hmm. Well, I actually planned on using Binz primer/sealer, which IS shellac-based, woth a white tint, since I'm going from a red finish to a lime green finish (working on a Green Meanie project). That way, I would have a nice white base, which to apply the color coat, without having the red bleeding through. I figured the adhesion promoter, whould help the new finish adhere, better.

Edited by Racer X
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If you need to strip poly off a body, Heavy Duty Circa 1850 was the only thing that even dented the finish on my Ibanez, which I was told by several members on PG was most likely poly. If you can't find it in store, it's like normal Circa 1850 but in a Orange can instead of tan. Works like a charm :D

Edited by Lord-of-the-strings
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If you need to strip poly off a body, Heavy Duty Circa 1850 was the only thing that even dented the finish on my Ibanez, which I was told by several members on PG was most likely poly. If you can't find it in store, it's like normal Circa 1850 but in a Orange can instead of tan. Works like a charm :D

I believe it is Kwik Kleen that makes a product simply called "Paint Stripper" It's sold by the gallon in an orange metallic can at Lowes (about $17) and probably other places. At work we use a Methyline Chloride overflow system to strip. It's effective, but has a huge overhead cost. The stuff i just named is the only thing that compares and in one area is better: It removes lead based paint with a vengeance. Anything that can remove lead can eat anything you give to it, even Cetol. This stuff will actualy penetrate latex gloves without eating them and give you chemical burns, so you know it works. The crude rule we use is that if it doesn't feel like its eating your flesh its not good stripper. The product you named is probably the same formula, i was just listing another alternative and my experience with it on actual 1850's paint, lol.

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