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newbie to poly

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kityuser

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ok so the beech kitchen door is made and I want to finish it with a satin finish water based polyurethane.

After the final sand I applied the poly and waited 6 hours for it to dry.

my question is this, how much do you need to sand inbetween coats, and how many coats.

I tried applying it spareingly and found that i was taking most of it back off with the rub down inbetween coats (using a very fine paper).

When I applied it liberally, I got a lovely deep shine, but marks in the finish. i.e. brush marks, swirls and bubbles.

how do you judge a happy medium, or where am I going wrong

regards

steve
 

Terry Smart

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Hi

I can only speak generally here without knowing more about the poly you are using (and even then I don't know a great deal about them!)

Firstly, I guess there are no instructions on the tin?

Sanding between coats is just to smooth the surface to achieve a finish as close to that of the bare wood as possible. You should be using a fine grit paper (usually the same as you finished the wood with) and sanding quite lightly. If possible you don't want to go through the finish itself.

Several thin coats are going to be better than one thick coat.
Normally with polys the first coat is thinned to act as a sealer but water-based finishes don't like being thinned as much as would be needed here.

I'd suggest giving the poly a very light sanding between coats and building up in thin coats... which is pretty much what you are doing I think.

I hope this gives you some pointers!
 
A

Anonymous

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HI Steve

I usually put on a sealing coat and lightly de-nib and then 3-5 thin coats without sanding between them. I sand after these are dry with 600 grit wet and dry (usually wet) and then apply the final coat - very thin again


Hope this helps

Cheers

Tony
 

Alf

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Steve,

I've found putting on a couple of thin coats and then lightly sanding quite effective. You don't want to sand too heavily, more like wiping a table than scrubbing a floor :wink: ; you're just denibbing really. Can't remember off hand, but I think last time I used 320 grit (?) round a cork block, although my old stand-by "Webrax" is also very good, especially for non-flat surfaces. Take extra care you don't sand right through at the edges; it's all too easy.

Cheers, Alf
 

Aragorn

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Nomenclature question: Isn't "water-based polyurethane" just acrylic varnish?
Doesn't a "polyurethane" by definition have to be oil based?
Just wondering!

For water-based varnish I apply up to 5 coats, with a very light rub down in between all coats (about 400 grit). I don't worry at all if I go through the finish to the bare wood on the first coat or two, as I see this as being a part of levelling the surface, especially if the grain has been raised by the first coat. Whenever I do go through the surface, I would always make sure to apply 4 more coats from then on.

Secondly, where poss I apply water-based varnish with a roller. Doesn't leave any brush marks etc so fewer rub-downs/re-coats are required. Normites favour those foam brushes :roll: supposedly for similar reasons. :wink:
 

Chris Knight

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Aragorn,

Acrylics and urethanes are different resins. "Water based" is actually a misnomer, these products are water-borne. In other words they are resins in suitable organic solvents with additives to allow them to be carried in a water medium - usually highly emuslfied (which is why you can't thin them with water like you can thin an oil based finish with say white spirits - the emulsion separates as the proportions get out of hand).

The chemistry behind these things is very complicated but formulations seem to be improving all the time.

The primary motivation behind these water-borne finishes is to reduce VOCs and keep us cool in the future.
 

frank

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steve use some good foam brushes i found them better than paint brushes and finish off brushing one way i used wet and dry paper to denib between coats.
 
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