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Nitro Over An Old Polyurithane?


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I'm revisiting my old guitar (see what I'm talking about in This Thread) and it has a polyurithane finish that I brushed on way back when. However, I'm going to be re-doing it with a black back and sides, and then clear over it all (this time all the paint will be nitro in rattle cans). My question is do I need to get rid of ALL this past poly then grain fill (cause it neverw as in the first place, gunna use Devcon epoxy) and then shoot my nitro, OR should I just rough up the poly, grain fill it, and then shoot the nitro over the poly?



PS: The poly finish is almost a year old, so it's fully cured obviously (if that makes any difference)

Edited by verhoevenc
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I,m a newbie but I used to work for a paint company

You will need to strip all the old Poly off either by sanding or by using non caustic paint stripper. The latter is messier but would be a lot quicker

WARNING- if you use paint stripper be very wary of your glue joints. Put it around but not over the top of them. Scrape off the stripperwith a blunt scraper

Wash down thoroughly with Metholated Spirits as this evaporates quickly and will help draw out any remaining stripper in the pores of the timber

Give the guitar an overall very light sand, dust off, tack rag then your ready to grainfill and topcoat


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Another newbie here but I think you need to get all the old poly of because laquers react badly with poly. Really should be done well because the smallest bit of bubble or wrinkle will kill the buzz, if you know what I mean. 3-4 days to get a non-newbie reply is fairly common so I think I would wait for a finish-veteran to chime in. That epxoy filler is like really hard to use, isn't it? I have never seen it used or anything but the tutorials on it sound like a long process. Just my thoughts., FWIW.

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Don't feel too bad. I've been around for a while and I'm rapidly losing interest in this forum because of the juvenile combatative attitude that a lot of the children here have.

Nitro will not go over poly and give you a finish to be proud of if it works at all.

Strip it to the wood and start over.

I am less careful with glue lines. I've refiished literally thousands (yes thousands) of pieces of furniture and as long as you don't submerge your piece in caustics and leave it there for hours you should have no trouble.

That said, The caustic stripers are the fastest way to go but the chemicals are nasty. The citrus based strippers are a ot slower but do a very good job, especially on something as small as a guitar.

Poly is a pain ion the butt to strip and I reach for the marine grade stripper. I also have no feeling left in the ends of my fingers from chemical exposure.

Strip it, sand it. and follow the tutorials here and at Re-Ranch. There are some good posts on the pros and cons of various grain fillers.

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Yeah, i ended up just sanding the stuff off the other day with my palm sander and doing it by hand for the small corners, etc. I put my first layer of Devcon epoxy for filler today. I say first cause it'll probably take more than one time AND because putting on the epoxy has shown me the places where there is still poly (quite handy IMO). It's darkw here I got it all off, and then lighter where there is still poly on the wood.

I'll let ya'll know how this turns out.


(Gunna do it with some reranch black for the back and sides, and then ordered some DEFT lacquer in a can for the gloss)

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