How can I stop pinholes from forming in my varnish?


  #1  
Old 08-22-06, 03:40 PM
H
Member
Thread Starter
Join Date: Aug 2006
Posts: 17
Upvotes: 0
Received 0 Upvotes on 0 Posts
Question How can I stop pinholes from forming in my varnish?

I'm currently making a coffee table in the laminated plywood tradition of Kerry Vesper. Having filled, sanded and stained the wood to satisfaction, I've begun varnishing the surface.

Because I'm using endgrain plywood, small pinholes are all over the place. In retrospect, I should have skimmed the entire thing with plastic wood or something before staining, but I was counting on the varnish doing a better job of leveling the final surface.

Currently the table has 3 full coats of satin urethane varnish on it, and it's still covered in pinholes. The small depressions and cracks in the plywood aren't really filling with varnish, it seems like there's surface tension or something causing it to bead up around the edges, but not penetrate into them. I use a 2" high quality bristle brush to apply it (straight, I only thinned the first coat), and while it's wet, the film covers 100% of the surface. As it dries, the pinholes reform while varnish builds up everywhere else.

Could it be my varnish of choice? Right now I'm using the low-tech version, as in, slow drying urethane. There are fast drying versions, but I wasn't as happy with their account of 'satin' (they seemed too glossy). If it would fix my problem, I would switch though
 
  #2  
Old 08-22-06, 07:13 PM
C
Member
Join Date: Nov 2001
Location: Taylors, SC
Posts: 9,261
Upvotes: 0
Received 0 Upvotes on 0 Posts
At some point you would need to sand the coat out smooth, taking down the finish until the depth of the pinholes you have. This will essentially fill the flaws in the surface. Your next coat should cover this smooth surface smoothly.

Hope this helps.
 
  #3  
Old 08-22-06, 07:41 PM
XSleeper's Avatar
Group Moderator
Join Date: Dec 2004
Posts: 27,318
Received 1,979 Upvotes on 1,774 Posts
>>Having filled, sanded and stained the wood....

I thought you said you filled the wood prior to staining? So where are these pinholes coming from- they should have all been filled in? Are you sanding with 220 grit between coats? Was the filler completely dry before staining? Was the stain completely dry before varnishing?

Regarding some "satin" finishes, many of them take several days or a week to dry down to a satin sheen. Until then they will appear glossier than normal.
 
  #4  
Old 08-22-06, 08:15 PM
H
Member
Thread Starter
Join Date: Aug 2006
Posts: 17
Upvotes: 0
Received 0 Upvotes on 0 Posts
The plywood is 3/4" spruce select. About 40 strips turned on their sides and glued together forms the surface. Being pretty much the worst plywood in the whole yard, the spruce is full of defects. Like I said, most of the major ones were filled, but there's all sorts of tiny imperfections that appear when the varnish dries (and I figured a satin finish would disguise a lot of them).

I have been sanding between coats... steel wooling with #000 actually. The steel wool seems to knock down the surface a bit more uniformly than sandpaper. I always wipe the surface with paint thinner, let it dry, and tack before applying my varnish.

At this point, it seems more like the pinholes are 'resisting' my varnish. I might venture the guess that I need something with a higher build, or higher solids or something that can 'bridge' the small gaps (or at least not bead up around the edge).

For curiosity's sake, here is an image of the table before it saw any stain or varnish. I made the table's base with aluminum stock (I have a lot more experience with metal and automotive coatings)

http://img.waffleimages.com/img/ca4112239bf0fbf5334de49c49134ecf81111afa/table_prestain.jpeg
 
  #5  
Old 08-22-06, 11:50 PM
J
Member
Join Date: Jun 2005
Posts: 184
Upvotes: 0
Received 0 Upvotes on 0 Posts
Paint thinner takes time to dry. If you're not waiting long enough for the thinner to evaporate the pin holes may just be the thinner off gassing. If possible experiment on a small area and skip the thinner.

Jan
 
  #6  
Old 08-23-06, 03:40 PM
C
Member
Join Date: Nov 2001
Location: Taylors, SC
Posts: 9,261
Upvotes: 0
Received 0 Upvotes on 0 Posts
Steel wool may abrade the surface more uniformly, but it does not make it flat. Making a flat surface, a uniform surface, will help with smoothness and eveness of the finish. Make sure you are not overbrushing the varnish, causing air bubbles to be entrapped in the finish as this can present similar results.

Making a smooth top of the strips in your picture would be a herculean task without generous use of fille in the beginning.

Hope this helps.
 
  #7  
Old 09-05-06, 06:59 PM
C
Member
Join Date: Sep 2006
Posts: 2
Upvotes: 0
Received 0 Upvotes on 0 Posts
sealer

The biggest reason I can see for the problems you are having is that you are using a very slow drying finish over a porous surface. It's probably hapening over every other layer of the plywood, where the endgrain of the wood is showing. It's like a bunch of little tubes slowly siphoning the finish into the wood. Something you might want to try is using a wax-free shellac to seal the wood. Shellac will fill these voids much more quickly, since it dries quickly and will not have as much time to be absorbed, and can also be re-aplied many times in a short period. This will also help limit the need for a skim of filler over the entire surface which from experience, does not leave very desirable results on the endgrain of softwoods. The shellac can then be topcoated with the poly you are using. Shellac is also a great bond coat which can be used over an (cured)oil based finish to bond a water based one to. In the same way, you can flatten your existing work with some 220 grit paper, use the shellac, sanding between every 2 coats or so and turn back to the poly for your final finish. As far as sanding, use 320 grit paper, with a sanding block between coats. Save that 0000 steel wool for a month or so, to go back with a little paste wax to smooth out the finish. When doing this, rub with firm pressure to keep your satin sheen.
 
  #8  
Old 09-09-06, 10:10 AM
H
Member
Thread Starter
Join Date: Aug 2006
Posts: 17
Upvotes: 0
Received 0 Upvotes on 0 Posts
These were all great posts. What I've learned for next time:

1) Skim coat of wood filler before staining
2) Use a sanding sealer or shellac first

The table's done now, I finished it and now I'm back at university, so it'll be another 8 months before I can get back into the shop . I've got pictures of the end result to show off though
http://img.waffleimages.com/img/a351bdd585c4f41e88cd26db4413c2bf2e56838f/table_small.jpg
http://img.waffleimages.com/img/6ad807ecf4534ac860a0f4bde87f2b141e215789/table_small2.jpg
http://img.waffleimages.com/img/e514136beb9f138147ff5eae993ff4ab9166e688/table_small3.jpg
 
 

Thread Tools
Search this Thread
 
Ask a Question
Question Title:
Description:
Your question will be posted in: