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| Question for Filippo http://www.luthiersforum.com/forum/viewtopic.php?f=10101&t=40308 |
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| Author: | Pat Foster [ Thu May 09, 2013 3:59 pm ] |
| Post subject: | Question for Filippo |
Filippo, A few months ago, if I recall, you were having finish issues where there were places where the Zpoxy - or System3 - wasn't adhering. I never did quite get what you were experiencing. Was that what you had? There wasn't a problem with the first coat on bare wood, only the second coat, after sanding back just to the bare wood. Looks like it's trying to bead up. Anybody else? TIA Pat |
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| Author: | George L [ Fri May 10, 2013 8:40 am ] |
| Post subject: | Re: Question for Filippo |
Wish I could help you, Pat. Best I can do is give this a little bump. |
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| Author: | Stuart Gort [ Fri May 10, 2013 9:46 am ] |
| Post subject: | Re: Question for Filippo |
Hey Pat...I did some panel tests that revealed an adhesion issue with nitro over Z-poxy. The thorough solution was: After the final scuff of the epoxy give it a good cleaning with naptha and then spray a coat of vinyl sealer prior to the lacquer topcoats. Let the vinyl sit overnight then scuff it with 240 (actually I use old 150 because it doesn't clog nearly as easily as 240) and then spray the lacquer topcoat. There's a significant difference in adhesion between using or not using the vinyl. One point....although it's called vinyl it's really a type lacquer...so the lacquer topcoats do burn into the "vinyl". You're welcome to visit my shop anytime...btw. It'd be nice to have the visit. I'm right in the middle of some adhesion tests now. |
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| Author: | Pat Foster [ Fri May 10, 2013 10:55 am ] |
| Post subject: | Re: Question for Filippo |
Filippo, Maybe it was someone else. Brain @$%$ of the aging and all that. My recollection is a bit clearer after thinking about it. There were small areas in the finish that weren't wetting/bonding properly to previous coats, showing up as light spots under the top coat, in the pores. Thanks for the bump, George. This particular problem is with the second coat of Zpoxy. It's like when you wash a car that has a good coat of wax, the water is repelled, so it beads up and runs off, but here instead of beads, there are spots where the Zpoxy is repelled, leaving voids in the second coat. I suspect some sort of contamination even after sanding back. I've since scraped it all off and am starting over, but I'm still wondering what it could have been. Starting over after two coats of Zpoxy is something I'd like to avoid in the future. Stuart, A visit would be great. Maybe this weekend. Thanks, all. Pat |
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| Author: | Stuart Gort [ Fri May 10, 2013 11:36 am ] |
| Post subject: | Re: Question for Filippo |
The beading up can be caused by a few things...obviously contamination of some sort could have been introduced onto the surface but I think that it's more likely to be a migration issue. Epoxies of all types cure based on cross-linking. If you get too much of the part B in the mix...after the cross-link...unlinked material can often migrates to the surface. You end up with a kind of greasy film on the surface. An even film could indicate too much part B. A blotchy effect could indicate not mixing the material well enough. Z-poxy is pretty forgiving on the ratio...but I still weigh it when I use it. Prior to sanding scrub with naptha or alcohol....and THEN sand. Just make that a habit. If you have any migrated material on the surface and then sand prior removing it...it gets everywhere...in the paper and all over the surface. Naptha....sand....naptha....ought to take care of this completely. Fresh epoxy won't cross-link to previously cured epoxy bit it is very aggressive and provides a "super-bond" mechanically to an older coat...if it's perfectly clean. |
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| Author: | Pat Foster [ Fri May 10, 2013 11:54 am ] |
| Post subject: | Re: Question for Filippo |
Zlurgh wrote: The beading up can be caused by a few things...obviously contamination of some sort could have been introduced onto the surface but I think that it's more likely to be a migration issue. Epoxies of all types cure based on cross-linking. If you get too much of the part B in the mix...after the cross-link...unlinked material can often migrates to the surface. You end up with a kind of greasy film on the surface. An even film could indicate too much part B. A blotchy effect could indicate not mixing the material well enough. Z-poxy is pretty forgiving on the ratio...but I still weigh it when I use it. If you suspect an issue (or just make it a habit)...prior to sanding scrub with naptha or alcohol....and THEN sand. If you have any migrated material on the surface and then sand prior removing it...it gets everywhere...in the paper and all over the surface. Naptha....sand....naptha....ought to take care of this completely. Fresh epoxy won't cross-link to previously cured epoxy bit it is very aggressive and provides a "super-bond" mechanically to an older coat...if it's perfectly clean. Stuart, The more I search around, the more it seems like contamination, amine or otherwise. I came across this thread from a while back, which leads me to believe it could have been the age of the Zpoxy. I had that in mind when I was mixing it, thinking well, just one more time and I'll toss the rest. I'm also thinking I might need to pay more attention to mixing ratios, even though Zpoxy is said to be less sensitive than System3 SB112. I'll remember that scrub before sanding. The bond where the top coat had not pulled away was solid. I did note that, when comparing the heights of the last few ounces in the bottles, they were identical so I haven't been too far off. But still, it's not the same as weighing. Thanks, Pat |
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