Help! 2nd Crack At Finishing..

[Appel]

I am trying to refinish a les paul copy. Did a bad refinish way back when. I stripped it with chemicals, sanded it down, wiped it down with mineral spirits, stained it and did a black sunburst, using a technique I found on this site. Everything looked great, but when I started putting the first round of polyurethane on it I noticed that I could see the sanding marks in it. I went from too coars to too fine in one step. This was a day or two after putting the polyurethane on it, so I just grabbed the mineral spirits and stripped the front down again (the back seemed fine). I resanded, stained, and did my sunburst again. I got my first coat of poly on there and everything seemed fine. I waited about 6 days, sanded with fine/ultra fine wiped it down with tack cloth, and a quick wipe down with a damp rag and started in with the 2nd round of poly. After about 45 minutes or so, I went in for another coat and I noticed a couple of spots where it looked like the poly was bubbling up along the edges. I sprayed the 2nd round, hoping this might be something I could sand out of the final finish, but when I came back about another half hour or so later, much to my horror the bubbling had spread about 3 inches and looked terrible. I went so far as to try and scrape off the poly and it was taking everything off, down to the wood grain. I'm back at square one! But that doesn't bother me as much as not knowing what the hell I did to cause that!? Was it the moisture from the damp wipe down? I thought it was pretty dry from that, and why would it take EVERYTHING off? I need help!

[Appel]

I am trying to refinish a les paul copy. Did a bad refinish way back when. I stripped it with chemicals, sanded it down, wiped it down with mineral spirits, stained it and did a black sunburst, using a technique I found on this site. Everything looked great, but when I started putting the first round of polyurethane on it I noticed that I could see the sanding marks in it. I went from too coars to too fine in one step. This was a day or two after putting the polyurethane on it, so I just grabbed the mineral spirits and stripped the front down again (the back seemed fine). I resanded, stained, and did my sunburst again. I got my first coat of poly on there and everything seemed fine. I waited about 6 days, sanded with fine/ultra fine wiped it down with tack cloth, and a quick wipe down with a damp rag and started in with the 2nd round of poly. After about 45 minutes or so, I went in for another coat and I noticed a couple of spots where it looked like the poly was bubbling up along the edges. I sprayed the 2nd round, hoping this might be something I could sand out of the final finish, but when I came back about another half hour or so later, much to my horror the bubbling had spread about 3 inches and looked terrible. I went so far as to try and scrape off the poly and it was taking everything off, down to the wood grain. I'm back at square one! But that doesn't bother me as much as not knowing what the hell I did to cause that!? Was it the moisture from the damp wipe down? I thought it was pretty dry from that, and why would it take EVERYTHING off? I need help!

A buddy of mine claims that this happened because of the oil off my fingers combined with the damp wipe down. He said I should have done a wipe down with mineral spirits after the fine sanding in between poly coats. I was afraid the mineral spirits would break down the poly. Anyone else want to weigh in on that?

[egdeltar]

Definitely sounds like some sort of contaminant. Or you possibly are spraying to heavy and it is gassing off.

[Appel]

I don't think it was too heavy a coat, I was trying to be careful about doing multiple light coats. Plus it was a LARGE area, right along the cutout. I think it was a contaminate, too. I was advised to use lacquer thinner instead of mineral spirits, as spirits is too oily and not as volatile.

[Southpa]

Are you doing conventional spraying or rattlecan? Rattlecan has higher solvent content and it could affect underlying coats, even if poly, under the right conditions. Another factor might be temperature and ventilation, especially with things colored black. Black absorbs heat more than anything. I've had massive bubbling occur from spraying an already hot finish. The clear skinned over real fast and couldn't gas off. The bubbles formed, some up to 1 cm across. Any one of those factors can give you trouble. If you are using rattlecan poly then drying time should be longer because of more solvents needing to gas off. Thinner coats are also wiser, once again, because of solvent content which might reactivate previously laid coats if not fully cured.

[kpcrash]

Also - don't forget - if steel wool is involved anywhere, unless it says "oil" or "lube-free" on the package - it's oily.

[syxxstring]

Are you following the finishing schedule recomened by the manufacturer? Not some made up guy's idea on the internet. Are you staying within product lines?

[Appel]

Yeah, the poly mgf's instructions say to coat within two hours or wait five days and sand in between. The area seemed to be within temp range and I gave it at least 6 days for drying time.

[kpcrash]

One more thing... are you using Wet/Dry sandpaper or just regular dry grit? Asking because most wet/dry also contains a lubricant that can sometimes foul up finishes if not wiped down enough (like with naptha) before recoating.