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I know there are types of finish, like nitro, where new layers will blend into old layers for invisible repair, and some that don't, and leave witness lines between layers.

What does polyurethane do? And is there a thickness limit like some finishes, where if you build past x, it will check under it's own stresses?

No burn in with polyurethane. That is why you have to sand between every coat.

But, does sanding surface prep prevent witness lines?

Anytime you choose a mechanical bond over a molecular bond, you risk witness lines. If you go this route, make sure you apply enough layers that will bond on a molecular level to avoid finish sanding into the mechanical bond.

This means, if you want to avoid a heavy finish, you got to more than scuff the finish. At the same time, you don't want to hit wood or epoxy.

I've had mixed results with this. Best to set aside a good long day when doing ur spray ups.

Of course not. In fact it practically guarantees witness lines. Another reason I don't use poly.



As I said, polyurethane does not burn in so a molecular bond is simply not possible.

My personal experience with polyurethane is limited to floors, and the instructions said that if the second coat was applied within 8 hours, no sanding was needed.
To me this implies there's some degree of molecular bonding if the previous coat has not cured too much.
I let it dry for several days between coats, and sanded between coats, so my only real information is from the instructions on the can.

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A man hears what he wants to hear, and disreguards the rest. Paul Simon

I am sure there are differences among the various brands, manufacturers, and finish formulations, but generally, you have to sand between coats.

check the manufacturers data and info sheets on the product .. some poly's allow you to recoat 3-4 times within a given period, usually around 45-60 minutes, and those coats will fuse as one ... then let cure overnight, scuff sand and spray another set of coats ...

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Tony Karol
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"let my passion .. fulfill yours"

Polyurethane and polyester can both be spot repaired successfully. Since both of these type of finishes usually have a chemical catalyst or hardener to initiate a full cure, the only way to achieve an acceptable repair is by a mechanical blend. Granted, it's not easy and may require several attempts with various means of abrasion and buffing of the blended area but if a spot repair is the best repair for a particular situation then it's worth the extra time to experiment. One of the first things I did before fully switching to all polyester finishing was to learn how to do a spot repair.

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My major concern is building a gloss coat, then having witness lines from coat to coat in the final result. Test panel time I guess.

Ed, there is a window of time you can build coats without sanding and the finish will act as one coat when dry. I used to spray varnish and polyurethane on kitchen cabinets this way. Hundreds of them. Use a thinner mix. Spray your first coat, let it flash off and set for a bit. The solvents in the next coat will re-activate the prior coat and they will cure/dry together. You're not letting the prior coat dry, just flash-off and set-up to finger-touchable). If you want to do a level sand, spray a few in this method, let it dry overnight or so, level and build the final however many coats you need, one on the other as I've described. Each series of builds will dry together. You'll need to spray lighter than normal and thinner than normal coats. I've never had witness lines this way. Of course, if you build 3 coats, let dry over night, and build 3 more, you'll have a witness if you sand through the final 3.

I have no spray equipment. Some sort of hand application will have to do...

I padded poly on a dining room table, almost like a French polish. I cut it by about 35% with thinner, and did 3 coats a day for 3 days with just a light wipe with 220 paper between sessions, and a hand buff with a old t-shirt to finish. Of course a table isn't a guitar.

Alex

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"Indecision is the key to flexibility" .... Bumper sticker

My comments were rhetorical. Yet, witness lines are caused by sand throughs. If you mechanically bond a layer, then add many more, and you did not have witness lines to start with, you can end up with a reasonable finish. But this is hard to do right.

Most of the modern water based Poly's can be redissolved with denatured alcohol. You wipe it on, let it penetrate for a few minutes, and then you can add coats that will fuse to what you had already applied. It is also how you can repair them. In repair situations you may not have the needed information to what finish you are working on, and in building you may still have other problems if you are not spraying the finish; my dilemma too Ed Bond!

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If what you see is what you get, then Stevie Wonder ain't got nothin'!

Glen, is this applicable to all polyurethane formulations, or have you done this with just one particular brand. General statements can sometimes be misleading.

I've used more than one brand. Best way to find out if it works on a brand is try it on scrap.

For the water based poly like General Finished Endurovar, would you suggest thinning it with water or would thinning with denatured alcohol cause a "better bond"?

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Like Roger mentioned if you do the additional costs within a certain time frame no need to sand. My experience is with the oil based only. Not sure about the water based.

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Another day, another dollar.

I use the General Extender to thin Endurovar, and other General finishes and stains. It gives a little longer wet time to level when spraying finish, and longer wet time for stain if you are wiping it off.

Alex

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"Indecision is the key to flexibility" .... Bumper sticker

I have never tried Endurovar, but the alcohol will dissolve the underlying finish enough to allow for cross linking. It is better to wet it down once, and not repeatedly, or else risk smearing and working in contaminants like dust...

Thinning is normally not needed with most products until the last set of coats. It is used for better flow out, so you don't have to do much level sanding, which will reduce the risk of sanding through layers and creating witness lines. But it really depends on the product more than anything, and what the manufacturer has to say about their product.

Just don't ever use tap water, as it contains minerals and other contaminants that will screw up the look of the finish. Only distilled water will do, with well cleaned containers and other equipment.

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If what you see is what you get, then Stevie Wonder ain't got nothin'!

I myself am embarking on a new adventure in finishing, as I have one of those crazy 7string basses made of 4 woods with varying properties here that the client wants dyed blue. I am using KTM-SV for the first time, the KTM factory burned down so it is not available for the time being, and if I screw up it will become a nightmare! Top that of with the fact that I don't have nor can I afford (my lotto ticket was a dud) spray equipment right now, making the process all that more challenging!

My main concern right now is whether using the KTM-SV with a wipe or brush over the dyed wood (TransTint) will be problematic, so anyone with experience: please chime in.

I can only proceed with caution and hope the finishing gods are on my side!

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If what you see is what you get, then Stevie Wonder ain't got nothin'!

I have tried wiping Enduro-Var with DNA. It does make the surface "tacky". But it will not ensure a complete molecular bond. If you go this route, and there are no witness lines currently, then you should do whatever you want to prep surface (sand, dna) then apply enough sequential coats that would aid in avoiding a sand through at the mechanical bond layer.

If you already have witness lines, you always will. Unless you sand back and start over.

With EnduroVar, there is a 24 hour recoat window where sanding between coats isn't required. Whether this is a mechanical or molecular reason, I don't know. For cabinetry, I can usually get three coats done in in one day, with a scuff sand after the second coat.

Alex

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"Indecision is the key to flexibility" .... Bumper sticker

Each chemical system you use has its own properties that are, or are NOT, tested thoroughly by the people that make them. Chemically, polyurethanes cover a wide range and can be consistently easy to use or be very difficult. The companies that make chemicals can be very thorough developing them, or very sloppy. A good company provides clear instructions and a stable, consistent product.

I can give you dozens of example of companies that do not test their products and leave it up to the customer to figure out how to use them. There are also companies that do an extremely GOOD job of testing their products and laying out procedures for using them. When you find one of those companies...it's a good idea to stick with them and investigate them first if they have something else in their product line you can use. Life is so much easier when you have people on your side.

There are urethane coatings that stay open (meaning that the surface molecules remain receptive to co-linking with subsequent coats) for many hours. But because the process of curing is a chemical process and once a full cure is achieved the material is inert and will no longer bond chemically. Mechanical bonding is characteristically MUCH weaker than a molecular bond and witness lines may occur when sanding through layers.

Urethane is really meant to be applied all at once. This is, of course, far less handy than nitro, but once you get the proper amounts to mix figured out, and are willing to hassle this mixing issue, the benefits of catalyzed coatings far outweigh the ease of nitro finishing.

Personally, I use catalyzed polyester now, primarily because sprayed polyester sanding sealer (a pore filler) is so stinking fast and easy that I'll never go back to any other finishing method. Add to that...one can approach a piece of bare, sanded wood, and have a polished, 100% cured, hard finish in two days.

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I read Emerson on the can. A foolish consistency is the hobgoblin of little minds...true...but a consistent reading of Emerson has its uses nevertheless.

StuMusic