Thread: Poly finish turns fingernail soft after rubbing out?

Sorry, but it is doing exactly what it was designed to do.

That's an exterior, marine grade varnish. It's supposed to be mushy so it doesn't crack to pieces and peel off when the exposed wooden parts expand and contract in the wet/dry cycles from stuff like a month of soaking rain followed by a month of hot/dry weather.

Stay away from exterior grade products like spar varnishes and boat products. I recommend that you pick an interior grade short oil varnish. These are designed to cure harder for furniture use. There are some pretty good hardwood floor and tabletop varnish products out there that work really well.

Likewise, stay away from oil products like tung oil and boiled linseed oil. These stay soft and mushy forever.

An interior duty short oil varnish is what you want.

Truckjohn is right. Marine varnish is the completely wrong finish for a guitar. Guitars are typically finished with a lacquer that sets very hard and can be polished to a high gloss. Something like Mirotone PC3220. If you don't have a good spray gun you can buy suitable lacquer in spray cans. It's an expensive way to go though. Alternatively you could finish it with super-blonde shellac.

I don't know what's available down under, but I like Minwax wipe on poly. I used to love Behlens rock hard before they changed the recipe. It's still a really good varnish, but the new VOC compliant solvents give me a ripping headache. Quite a few folks recommend hardwood floor urethane varnishes, as they build reliably and are very durable. Specific products currently available is probably a good question over on the Australia New Zealand Luthier's Forum.

Thanks guys - I’m disappointed because I made a point of going to a proper paint shop for advice (as opposed to going to Bunnings for a tin of Cabots) and they confidently handed me the Norglass.
So to salvage the situation - is another rattle-can poly going to be compatible over the top, or should I sand it back to bare wood? (Or is it going to depend on the product?)
Jack620 - I don’t have spray gear or booth etc. so it needs to be rattlecans, that I guess have pretty high-build and level out well, so I can have a fighting chance at rubbing out without getting witness lines/sanding through

I would cut it right back or remove the varnish completely. Then I'd use either Feast Watson Poly Varnish or Shellac. Hard Shellac will rub out better than Poly because it dries harder and is a harder film finish. Added to which you wont get witness lines as all coats of Hard Shellac melt into each other to form a single film.

I've recommended F&W Poly Varnish because of its high solids content. Also - whatever finish you use, keep it as low (thin) as possible, this will be less prone to scratches or for when you want to dig your nail into the surface!.

Originally Posted by wsal

I’m disappointed because I made a point of going to a proper paint shop for advice (as opposed to going to Bunnings for a tin of Cabots) and they confidently handed me the Norglass.

They obviously had no idea.

Originally Posted by wsal

So to salvage the situation - is another rattle-can poly going to be compatible over the top, or should I sand it back to bare wood? (Or is it going to depend on the product?)

It has to come off unfortunately. It's never going to be any harder than the softest layer. Weakest link, and all that. We don't know what brand this guitar is. Is it a cheapy, or an heirloom? That should inform your choice/cost of finish. Personally, I would not use any type of poly on a guitar I had gone to all the trouble of restoring.

For reference, this is 13 coats of Mirotone PC3220:

Thanks Jack - that’s a great looking guitar!
So the backstory is: it’s a Maton Mini 12 string that I picked up cheap with a bellied top, bridge lifting, and the finish on the top in generally poor shape.
So it’s not heirloom quality, and wouldn’t have come out of the factory with anything fancy (pretty sure these cheaper models aren’t nitro). I’m looking to just get a nice satin.
Speaking of Mirotone, any experience with Spraylac?

A Maton is worth doing properly.

Originally Posted by wsal

Speaking of Mirotone, any experience with Spraylac?

No, I haven’t. But it’s a pre-cat finish, so I suspect it’s very similar to PC3220. Maybe identical. You’d need a few rattle cans to do a guitar though.

Good advice above.

After removing the old varnish, there are two products that I would use in your situation; either:

• Wipe-on polyurethane, or
• Organoil hard burnishing oil.

WOP can be bought premixed, or just mix poly with an equal volume of turps. Organoil smells nicer.

The technique that I would use is:

1. Prepare surface, sand to 200 grit.
2. Wipe on a light coat of finish, wait 10 minutes, and wipe off with clean cloth,
3. Next day, repeat step 2 - (This fills the grain),
4. Next day, dip 400 grit sandpaper into finish and wet sand - surface must be damp all over, but not wet,
5. Next day, repeat step 4 but with 600 grit,
6. Repeat up through the grits - 1000, 1500, 2000, 3000 & 4000.
7. Allow to cure and apply furniture polish.

This will give a faux french polish - but very durable finish.

A very light touch is needed when refinishing, as guitar bodies are very thin. The wood generally runs 2-2.5mm thick. Veneered laminates generally have a 1/2mm top layer, if that. You absolutely will thin the plates, and can quickly sand right through veneer faces, leaving a mess, not to mention thinning the wood will change the tone of the instrument.

The other piece is that guitars are made of dissimilar woods. The top is made of one thing, while the back and sides another, and the neck yet another. You've also got rosettes, inlay, and binding, which may be 1mm thick or less. All these things sand and take finish at different rates. The factories have already factored all this into their finish schedules.

The surprise is how often you strip the even finish of a musical instrument and reapply new, and you end up with a blotchy mess due to the natural grain variation in the wood which was hidden by sizing agents, grain fillers, and stains.

Originally Posted by wsal

Thanks guys - I’m disappointed because I made a point of going to a proper paint shop for advice (as opposed to going to Bunnings for a tin of Cabots) and they confidently handed me the Norglass.
So to salvage the situation - is another rattle-can poly going to be compatible over the top, or should I sand it back to bare wood? (Or is it going to depend on the product?)
Jack620 - I don’t have spray gear or booth etc. so it needs to be rattlecans, that I guess have pretty high-build and level out well, so I can have a fighting chance at rubbing out without getting witness lines/sanding through

The fellows at the paint store simply won't know. If they did, they would have pointed you to a high quality, interior grade, high gloss urethane hardwood floor varnish. Those have to take high heel shoes and work boots and dogs nails clickety-clacking around, and they can't imprint where the furniture sits.

Unless you're already a champ with rattle can finishes, don't throw good money after bad. The typical rattle can clear finish is soft and mushy. You may have to get into expensive specialist products to find what you're after.

If you have a decent size city nearby, go visit a woodworking supply store. As them for a hard table top varnish product. Those finishes are designed to resist scuffing and scratches, and they're not mushy because if they are, the wife's vase leaves a ring imprint in the finish, and nobody wants that.

Maybe you can take it back to the paint store and show them the problem. At the very least they should refund the purchase price, if not also provide an appropriate stripper.
If they are serious they should contact the supplier tech advice line. Hopefully they will also learn.