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Well I done goofed up my polyurethane again :(

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Forum topic by GurfGurston posted 08-21-2018 02:01 PM 1106 views 0 times favorited 12 replies Add to Favorites Watch
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GurfGurston

9 posts in 336 days


08-21-2018 02:01 PM

I’m back with some pictures of the guitar and more questions. This is my very first wood working project so bear with me!

And thanks for the suggestions last time guys that was a big help getting the high spots down.

Some background to those interested: I’m building a guitar for my kids as a means to get into wood working (which I’ve been interested in for a while), and also to pick up my old hobby I was pretty heavily invested in until I graduated college and sold all of my equipment. But I want them to grow up around music and when they’re interested in my couch noodling on this thing they can have it for their very own. I bought this yamaha pacifica for 36 dollars and stripped it down of all the band stickers and other assorted “teenagering’s”. I went with a burned relic’d finish because it sounded fun and I didn’t think I could screw it up so bad… yet to add to it are some volume and tone knobs I built out of shotgun shells, and a backplate with the coordinates to my farmhouse so they don’t forget where they came from :)

I got the poly smoothed out and looking nice so I applied meguires car polish and buffed it with a rotary tool. Well, the rotary tools screw got ahold of the finish down towards the bottom and gouged it a little bit. So I grabbed some 400 grit wet\dry I had soaked overnight and sanded it down, wiped it off with a microfiber cloth, sanded more at 800 grit, then cleaned it a few times with mineral spirits. So it looked like I turned it from a scratch to a smudge. I had seen that happen to the entire body when I scuffed the poly between every few coats and a fresh coat of poly took care of it… but on my smudge it still hasn’t vanished after two fresh coats.

Should I sand it down some more and build that spot back up??


12 replies so far

View Aj2's profile

Aj2

2321 posts in 2217 days


#1 posted 08-21-2018 02:23 PM

Your messing with the wrong stuff if you want to buff out and level a finish.
Poly doesn’t play well .

-- Aj

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OSU55

2359 posts in 2409 days


#2 posted 08-21-2018 04:01 PM

Didnt see previous post – wb or ob poly? Have to agree luthiers usually use lacquer, polishes out much better. My choice would be solvent precat (several mfrs are good) or wb Target Coatings EM6000. Both need to be sprayed. The em6000 burns in like solvent lacquer so no ghost lines when rubbing out, and can be used to dropfill like solvent lacquer, both important finish characteristics when wanting a fully filled gloss finish. Whatever you use, let it sit for a couple of months, the longer the better, after you are ready to rub down for the final gloss finish. All finishes continue to shrink for quite a while, and have a bad habit of “telegraphing” – after a few months little waves start to show through – the wood grain telegrphing through to the surface. Its only noticeable close up with a raking light, but Ive had it happen with properly dried and prepped wood. Best defense is to let fully cure and shrink out completely.

View Planeman40's profile

Planeman40

1420 posts in 3180 days


#3 posted 08-21-2018 04:31 PM

Yep. The heat generated by power buffing is what created the problem. It softens the poly surface and the power buffing smears it around. You should have hand buffed. Slower, but it doesn’t generate the heat. Polyurethane is really a type of plastic. Sand down again, apply poly, let it dry absolutely hard (two days to make sure) wet sand again, and this time HAND BUFF using automotive buffing compound.

You learn by making mistakes. Keep at it though. I went through all of this too. I have been doing wood and metal working now to 60 years and I can’t tell you how many mistakes I have made over the years! I will say they are a lot fewer these days. Had you used lacquer or shellac you could have power buffed. However power buffing with an angle grinder is most likely too high speed. Most post power buffing of painted surfaces is done a low speed to minimize the heat build up.

-- Always remember: It is a mathematical certainty that half the people in this country are below average in intelligence!

View GurfGurston's profile

GurfGurston

9 posts in 336 days


#4 posted 08-22-2018 12:28 PM



Didnt see previous post – wb or ob poly? Have to agree luthiers usually use lacquer, polishes out much better. My choice would be solvent precat (several mfrs are good) or wb Target Coatings EM6000. Both need to be sprayed. The em6000 burns in like solvent lacquer so no ghost lines when rubbing out, and can be used to dropfill like solvent lacquer, both important finish characteristics when wanting a fully filled gloss finish. Whatever you use, let it sit for a couple of months, the longer the better, after you are ready to rub down for the final gloss finish. All finishes continue to shrink for quite a while, and have a bad habit of “telegraphing” – after a few months little waves start to show through – the wood grain telegrphing through to the surface. Its only noticeable close up with a raking light, but Ive had it happen with properly dried and prepped wood. Best defense is to let fully cure and shrink out completely.

- OSU55

Hi, not sure what wb or op is. I’m using satin wipe on minxwax poly.

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GurfGurston

9 posts in 336 days


#5 posted 08-22-2018 12:31 PM



Yep. The heat generated by power buffing is what created the problem. It softens the poly surface and the power buffing smears it around. You should have hand buffed. Slower, but it doesn t generate the heat. Polyurethane is really a type of plastic. Sand down again, apply poly, let it dry absolutely hard (two days to make sure) wet sand again, and this time HAND BUFF using automotive buffing compound.

You learn by making mistakes. Keep at it though. I went through all of this too. I have been doing wood and metal working now to 60 years and I can t tell you how many mistakes I have made over the years! I will say they are a lot fewer these days. Had you used lacquer or shellac you could have power buffed. However power buffing with an angle grinder is most likely too high speed. Most post power buffing of painted surfaces is done a low speed to minimize the heat build up.

- Planeman40

Thanks I’ll try lacquer or shellac next time for sure.

I sanded the spot down but still struggling. I’d hate to have to sand the whole corner down and redo it but I may be headed that direction.

View mahdee's profile

mahdee

4291 posts in 2187 days


#6 posted 08-22-2018 12:40 PM

WB Water base. OB oil base. I am pretty sure you cut through a layer of poly and it may take several coats to cover it up. However, by doing that you will build the poly on the surrounding area. I would sand the whole thing with 220 (smoothing and leveling the surface) before applying additional coats.

-- earthartandfoods.com

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GurfGurston

9 posts in 336 days


#7 posted 08-22-2018 02:36 PM



WB Water base. OB oil base. I am pretty sure you cut through a layer of poly and it may take several coats to cover it up. However, by doing that you will build the poly on the surrounding area. I would sand the whole thing with 220 (smoothing and leveling the surface) before applying additional coats.

- mahdee

Oh I get it now.

I’ll try sanding it all down and building it back up. I’m afraid that the haze is from smearing wax into the finish with my sandpaper. I cleaned it with a few applications of mineral spirits before I put more poly down in that one spot but it still just looks like a smear.

Do you think I need to sand down to the wood completely, reburn it if necessary then start building up the poly again?

View OSU55's profile

OSU55

2359 posts in 2409 days


#8 posted 08-22-2018 03:58 PM


Do you think I need to sand down to the wood completely, reburn it if necessary then start building up the poly again?

- GurfGurston

If you must use ob poly, no. Since you are rubbing it out, use regular poly thinned 10-20% and brush it on to get a thicker film. I prefer taklon brushes. Get several coats on within a couple of days and it will burn in, eliminating ghost lines. Best thing might be rattle cans of mw poly.

It would be better to strip and spray with a different product. Not sure the nc lacquer available in rattle cans would hold up – others can comment. If you have an air compressor, HF has an hvlp gun for $15 or so that will spray precat lacquer. For this application a large compressor is NOT needed – a small pancake type would do it, just have to wait a few seconds between passes for pressure to build.

View Andybb's profile

Andybb

1932 posts in 1023 days


#9 posted 08-22-2018 04:58 PM

If you want to avoid the fumes lots of luthiers spray Varathane water based poly. Sold at Home Depot. Thin it a little with water. (no more than 10%) Otherwise, I’d use Deft spray lacquer in the rattle can, also available at HD.

-- Andy - Seattle USA

View mahdee's profile

mahdee

4291 posts in 2187 days


#10 posted 08-22-2018 05:07 PM

No, I wouldn’t go to bare wood. Just enough sanding to have a even surface. If you do sand all the way down to the bare wood, then get a can of Deft spray lacquer and finish it with that. It dries in the mater of hours instead of days. Few thin spray at 1 hour interval should give a good finish.

-- earthartandfoods.com

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GurfGurston

9 posts in 336 days


#11 posted 08-23-2018 05:03 PM

Update: I don’t know why I thought the poly was thicker than it was but I made it to bare wood and rubbed off some of the burnt finish. When I built it back up I see that I fixed some of the pock marks from the picture above but it looks off to me so I’m going to sand it back down and torch the arm bevel, oil it and rebuild the poly since I don’t see any easy way out.

I find it pretty ironic that I started a project with the burned and relic’d finish theme and I’m being so OCD over this area when the entire guitar was burnt to a crisp earlier, haha.

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GurfGurston

9 posts in 336 days


#12 posted 08-24-2018 01:22 AM

Fixed, thanks fellas. Now to build up that poly again without screwing up :)

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