On wipe on poly and ghosting/witness lines

[russtolium]

Hey y'all,

I'm in the glorious yet tedious finishing stages of my latest build and keep running into a pretty frustrating issue. Using Minwax satin wipe-on polyurethane I've built a considerable number of coats on the body of the instrument and have things baby-smooth all over. The problem is that there are several large spots on the back of the body where I appear to have sanded through the top coat, even when starting with gentle 2000 grit wet sanding (see photo). This has happened with earlier coats as well in various places. I have taken care to apply with an even consistency, and have been letting it dry with the front-side up for 1:45 per coat when doing a series.

I'm trying to figure out how to go about correcting this, having recently educated myself on the properties of poly (and the general importance of thin coats), but I haven't found a huge amount of info on this specific issue. I know I have to apply another coat, but I don't want to waste time trying to apply the wipe-on the same way, resulting in the same thickness, resulting in the same issue.

What would you do to get a thicker final coat so this doesn't happen? Try a second application a few minutes after wiping the first on to get a thicker layer? Buy a can of brush-on and use that? Start at a grit even higher than 2000 when doing finish sanding or go straight to buffing? (there are usually still a few dust nibs to get out each time) Any thoughts from those out there who have fought this struggle?

 

[GMC]

How long are you leaving the poly to set between coats. It might not have cured fully and even ultra light 2000 grit sanding rips through it like 60 grit.

 

[russtolium]

How long are you leaving the poly to set between coats. It might not have cured fully and even ultra light 2000 grit sanding rips through it like 60 grit.

Usually at least overnight in a warm apartment, but I've managed to sand through even after days of curing.

 

[Lonnybass]

Lots of thoughts here. Firstly, Minwax stuff needs a solid 3 weeks to cure before it can be buffed. Dries in an hour, curing takes forever. And you want at least 10 coats of the stuff on there. 12 to 15 is better.

Here's the trick with that stuff...don't start the process with wetsanding. Sand as normal using gentle pressure with like 320 grit. You want an even white powder over the surface that you won't easily see while wet sanding. Once you're there, work your way up the grits from 400 to about 1200 to 1500. Then you can start wet sanding and polishing and it will start to look better. You can use a little buffing product after that to get more of a sheen.

 

[Beej]

How many coats is "a considerable number"? Using that wipe on poly, I've had best results after around 20 coats and let cure for around 3-4 weeks before going at it with sandpaper. I've also found the Satin to be a poor performer compared to gloss - gloss seems to go on thicker and stay that way. If you want a satin sheen at the end, you can scuff back gloss to make a satin finish...

 

[russtolium]

I've put over 30 coats on at this point, the wood pores have well exceeded the point of being completely filled leaving an extremely smooth feeling surface. I tried dry sanding at 400 several coats back and the issues were the same, I switched to wet hoping it would be gentler (and avoid corning, which it does nicely) and bumped up the grit considerably. I wipe dry and check my work often. If it weren't for the ghosting I could easily sand out the shiny spots left with 2000 grit so that everything was uniform.

I understand the final curing takes a while but how would that affect the fundamental issue that I'm sanding through the top coat? From my research no amount of curing displaces the fact that poly layers do not chemically melt into one another, so sanding through a layer after 24 hours of curing shouldn't be any different than after 3 weeks, right? As for waiting a few weeks to do buffing, that makes sense since there's friction heat involved.

 

[Lonnybass]

You're going to be removing the top coats for sure- that's the whole idea of the leveling and buffing stage. The idea is that you want to be able to remove an even layer so as not to have the lines. The issue with buffing layers that aren't completely cured is that you'll have the uneven removal issues that you'll just keep chasing.

I'd suggest switching the finish to gloss minwax instead of satin. It's going to be way easier to buff out an even surface if you can see the change in sheen. And work a little area at a time so you don't overdo it in any one spot.

 

[Beej]

Hmm, sounds like you're doing everything "right" compared to what I've done with wipe on. Are you using a block to sand? My only guess is that you might be using more pressure or sanding more vigorously than I do?

Waiting longer permits the finish to harden more - if its at all soft (compared to hard cure), the finish can ball up under sanding, and load the paper more than necessary, which can lead to more finish material being taken off while sanding than occurs with a really hard cured finish.

I hate to say it, but you might want to just keep doing coats and let them cure longer than our experience has shown us here before going at it like Lonny outlined above. I'm curious what your humidity and temperature are like where you are?

 

[russtolium]

The sheen on the satin is actually pretty easy to spot... but perhaps I'll consider a switch if one more pass with the satin doesn't work out. I've been using a standard pink eraser as a sanding block lately although previously I did it with just the flat of my thumb as backing. I may try to find a rubber block of some sort so I can try to spread the pressure a bit more evenly. I don't think I'm sanding with much force, just enough to remove the shiny spots. I have been dwelling on the same piece of sandpaper for entire sides of the body though, I might be more vigilant with switching the paper out since while wet sanding prevents clogging it ultimately does get a little bit of buildup.

I don't have any precise measurements as far as humidity goes but temperature wise it's in the high 60's/low 70's. Fairly high humidity outside right now but probably somewhere in the 50% range inside where the project lives. Warm winter weather for Virginia.

Maybe I'll just throw on another coat on and give it a week or so to cure more before another attempt...

 

[Beej]

Keep us updated. For what its worth, finishing is my least favorite part of the build and is typically the most work to get things looking stellar...

 

[Will_White]

Could it be possible that the satin isn't designed to be level sanded and buffed? Most of the time with satin finishes the last coat you put on is what you leave it as.

 

[SilGarzaJr]

Use micro mesh for glass like sanding. Up to 12000 grit.

Can be found at woodworker shops.

They start at 1500 and work up to 12000.

Works great!

Good luck!

 

[Means2nEnd]

I have done a bunch of wipe on poly finishes and Minwax by far and away has the least amount of solids in it which has good points and bad. I have found a minimum of 25-35 coats is need to build up at all. I have a question were you scuffing it or lightly sanding it between coats? If so this can greatly reduce how fast it builds up obviously. Cure time is important but but two three weeks is usually good. If you did 20 coats and just plowed them on one after the other every 1:45 min then it will take longer to dry because you are not letting each coat enough time to gas out and harden a little.

I do 25-30 coats and the most I will do in a day is 2. after hours of letting the coat surface dry I use 0000 Briwax wire wool the kind with no oil in it for furniture. You only need to delicately sort of brush the surface not take a bunch of material off only just scuffing the very surface. Then of course tac cloth it down or use a air compressor or both to get the wire wool dust off. So a good wipe on finish takes me about 3-4 weeks and I have never burned through even using a huge buffing wheel.

So not to be over critical but it sounds rushed. Too many coats too fast and not enough cure time. It's hard to completely diagnose there are subtle things that can make a difference. I has a similar experience but it was an adherence issue. I didn't use a sanding sealer so the finish never cured completely over the oily area and never could stick. I aways now after final sanding and dying and grain fill if that's done first I seal it with a half and half mixture of de waxed shellac and denatured alcohol this promotes excellent adherence.

Sorry I don't mean to do a tutorial but what helps me realize what most likely went wrong is to read how others have done things in steps and that seem to be somewhat successful and backtrack through what I did. Good luck...

I suggest next time use the General Finishes wipe on. It is a wipe on varnish to like the Minwax but the resin is all poly where the General I believe is 100% alkyd or at least mostly alkyd and some poly and has a much more natural less plastic look and feel and contains more solids so it builds easier and faster for next time maybe.

 

[russtolium]

Could it be possible that the satin isn't designed to be level sanded and buffed? Most of the time with satin finishes the last coat you put on is what you leave it as.

It doesn't really specify one way or the other. I'd love to just leave it as is but dust inevitably gets into the finish to some degree or another. If I give it another coat I may try just spot sanding the dust lightly and see if I can do the rest with polishing papers.

Use micro mesh for glass like sanding. Up to 12000 grit.

Can be found at woodworker shops.

They start at 1500 and work up to 12000.

Works great!

Good luck!

I actually tried this last night with a variety pack of polishing mesh I had lying around and made pretty good progress at polishing out the marks! I'm using a pink paper that's 4000 grit and I've probably made the marks 2/3's less visible so far. Fingers crossed that I can keep it up and remove them completely!

I have done a bunch of wipe on poly finishes and Minwax by far and away has the least amount of solids in it which has good points and bad. I have found a minimum of 25-35 coats is need to build up at all. I have a question were you scuffing it or lightly sanding it between coats? If so this can greatly reduce how fast it builds up obviously. Cure time is important but but two three weeks is usually good. If you did 20 coats and just plowed them on one after the other every 1:45 min then it will take longer to dry because you are not letting each coat enough time to gas out and harden a little.

I do 25-30 coats and the most I will do in a day is 2. after hours of letting the coat surface dry I use 0000 Briwax wire wool the kind with no oil in it for furniture. You only need to delicately sort of brush the surface not take a bunch of material off only just scuffing the very surface. Then of course tac cloth it down or use a air compressor or both to get the wire wool dust off. So a good wipe on finish takes me about 3-4 weeks and I have never burned through even using a huge buffing wheel.

So not to be over critical but it sounds rushed. Too many coats too fast and not enough cure time. It's hard to completely diagnose there are subtle things that can make a difference. I has a similar experience but it was an adherence issue. I didn't use a sanding sealer so the finish never cured completely over the oily area and never could stick. I aways now after final sanding and dying and grain fill if that's done first I seal it with a half and half mixture of de waxed shellac and denatured alcohol this promotes excellent adherence.

Sorry I don't mean to do a tutorial but what helps me realize what most likely went wrong is to read how others have done things in steps and that seem to be somewhat successful and backtrack through what I did. Good luck...

I suggest next time use the General Finishes wipe on. It is a wipe on varnish to like the Minwax but the resin is all poly where the General I believe is 100% alkyd or at least mostly alkyd and some poly and has a much more natural less plastic look and feel and contains more solids so it builds easier and faster for next time maybe.

Thank you for your insights man! Don't worry about the length. The frequency of coats varied throughout, I only sanded the following morning (or two) after a series of coats. Earlier on this was much more necessary since the wood is ash and it was only lightly grain filled, so there were a lot of gaps to fill. I totally agree that the devil is in the details and it's definitely going to help me plan future finishes (though I might switch to a nice lacquer for guitar bodies, haha).

I've been reading Understanding Wood Finishing and I know so much more now but even then it still only lightly covers issues like mine, just gotta crowd source it and learn from experience! Interestingly I did the neck in poly as well using Varathane in a rattle can and had no ghosting issues at all, even with how many times I had to re-sand it due to some overzealous spraying drippiness. The Minwax was actually a leftover from a table I re-finished and I opted for wipe-on over the spray Varathane because I could do it at home instead of the often-occupied spray booth at the makerspace I go to. Maybe I'd have never had the issue to begin with if I'd just used that!

 

[russtolium]

Update: The 4000 grit polishing paper did the trick! It took a while to polish them out but they removed the ghosting lines and I'm happy with the result. Onto buffing! Thanks for the advice everyone. Probably not gonna do another poly body for a while.