Help with using 2-part Polyurethane

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Old 09-08-16, 12:36 AM
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Help with using 2-part Polyurethane

Hi all. I'm in need of help & wanted to sign up after searching online.

I'm having a hard time trying to finish anything flat with an edge using 2-part Poly. I have used oil based Poly alot, but 2part just behaves so differently.
I'm using a product that you mix in a 1:1 ratio & doesn't require thinning, you just brush it on.

The problems are that there is always shrinkage right near the edge of the wood leaving the the coat thin & also it takes a considerable amount of sanding (240-400grit) to sand out defects like nibs, pinholes etc. With every test piece of wood I always end up sanding through the edges in various places before I can ever sand the whole surface dead flat.
This happens whether using grain filler or no grain filler on slightly porous hard wood, even if the coats are thin or thick. My work area is in temp range of the data sheet. But I have experimented in cold & warm & it's always the same pretty much.

My basic procedure is:

- fill grain with either thinned water based putty or thinned epoxy glue.
- sand raw wood dead flat with 180grit with a large flat sanding block.
- stain with spirit based stain.
- apply 1st thin (by 10%) coat of poly.
- apply up to 4 moderately thick coats with lite sanding in between each coat & a lite cleaning with alcohol or wax & grease remover.
-apply final coat thick.
-wait until cures, sand with 400 until mirror flat.

It's the last step that I can't complete, by the time I get the whole surface dead flat sanding out the defects like nibs I have cut into bare wood in various areas. Not to mention I haven't even worked up to 2000grit yet before the polish step.

I think the only thing I can do is avoid 2-part, I don't think its a problem with the brand even though it says 'Flooring' on the can. I think other brands would do this too. Wanting to see if anyone has had this happen & if you have any advice to get around these troubles. One thing I noticed is the sand paper under block tends to hug very slightly around he edge of wood as the block passes over the edge no matter what I do, so maybe this is adding to the trouble?

please help me. thanks
 
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Old 09-08-16, 03:29 AM
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Welcome to the forums!

The biggest advantage to using any 2 part coating is they dry to a more durable film. I'm not convinced you'd do any better with regular poly. You are over sanding it! Edges are the easiest to over sand. You should only sand it lightly or at least not enough to sand thru the poly. It may not be perfectly smooth but you will sand more after the next coat. Generally 3 coats will give a nice finish.

I normally start out with 150 or 180 grit for the first [sometimes 2nd] coat of poly and use 220 prior to the final coat. Generally anything finer isn't needed for any finish applied to wood.
 
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Old 09-08-16, 06:01 AM
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I suggest lightly breaking the edges with sandpaper if you are not doing that. Any finish adheres better if the edge is just slightly softened. If you are going for a mirror finish on furniture, say, I suggest you look at French polishing.

The best way to avoid problems with dust nibs is to use a fast drying finish; your poly takes so long to set tack free that it is bound to get nibs. Catalyzed lacquer would be another option.
 
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Old 09-08-16, 08:11 AM
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Too heavy of a coat will tend to exaggerate at the edges due to skin tension. Many thin coats are always better.
 
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Old 09-08-16, 11:58 PM
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Hi marksr.

The problem for me is that on the final coat things like nibs have to be rubbed out or else it won't be a perfect flat surface, & from using this there simply is no way to rub them out without cutting into the edges. 2-part for me anyway is so different with the surface tension problems, I never had this happen with oil based poly.
I'm convinced the only way to go is spraying with thin good atomisation which will prevent nibs & any major surface tension problems. To me there is simple no way to get a fine furniture finish with a brush.

Thanks.
 
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Old 09-09-16, 12:01 AM
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Hi XSpleeper.

Do you know if this skin tension is affected much by any high humidity?
 
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Old 09-09-16, 12:07 AM
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Hi CarbideTipped,

I was told that these surface tension problems are present with all 2part finishes. So I'm not sure Catalyzed Lacquer would be any better over spraying poly.

thanks
 
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Old 09-09-16, 02:47 AM
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Spraying always gives a finer finish. It is always important to apply the finish is a clean environment! While an exhaust fan can be helpful while the coating is being sprayed it's generally better to have no air movement once it's been applied.
 
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Old 09-09-16, 07:44 AM
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The reason I suggested catalyzed lacquer is that it is very tough, and it hardens very fast so there is very little window for dust to settle and stick on the finish. If you break the edges just slightly the finish will hold on them.
 
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Old 09-09-16, 08:55 AM
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"Break the edges slightly" means lightly rounding the corners with sandpaper as you prep the wood, before you stain. Doing this first helps ensure that you don't ease the corner as you sand the finish later on. It also reduces the humping you get from skin tension by eliminating the sharp edge that causes the finish to taper to nothing.
 
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Old 09-12-16, 11:55 PM
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Hi Xsleeper,
I tried rounding the edges a bit but the clear still thins near the edge. I think if I round it off enough it will look bad as I like the sharp edge. I think I'm better off brushing an extra amount near the edges, i read that adding a bit more near the edges is easier when sprayed.
 
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Old 09-13-16, 03:18 AM
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Try sanding less or sanding lighter at the edge. If that doesn't get it slick enough, apply another coat and go from there.
 
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Old 09-14-16, 12:09 AM
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Hi marksr

Applying another coat after the previous is dry will lead to witness lines being seen when sanding.

You simply can't sand lightly, as the aim is to get the whole surface level & that requires going over it all with a flat block.
 
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Old 09-14-16, 03:50 AM
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Continuing to sand thru the poly between coats is not the answer! If you build up several coats [sanding lightly in between to promote adhesion and remove minor defects] you can then block sand without as great a risk of sanding thru the poly.
 
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