Thanks for everyone's input. To address a few questions:
- The polyurethane is oil-based.
- I've been careful not to paint the polyurethane on, and go over it once at
the end in strokes that span the length of the piece.
- They're definitely bumps, not craters. If I scrape them off, they're
polyurethane chunks.
- The bumps are quite numerous, and especially stand out in certain
lighting.
- I don't think I'm using the wrong brush, as it said it could be used for
oil-based polyurethane.
Some things that were said that may have a high probability of me screwing
up:
- Contaminated polyurethane. I don't remember these bumps on the early
coats when the can was new, but I didn't really pay attention either.
Also, I did inadvertently leave the lid off for several hours.
- It might be dust. I'll clean up a little better this time and see if it
makes a difference.
- I may be working the polyurethane too much as stated by Mike Marlow.
Larry Jaques, you recommended a "real finish" - what were you referring to?
I noticed that they have expoxy coatings, but the stuff I saw at Home Depot
was $25 for a quart that covers four square feet. Rather expensive.
Thanks again!
--
Michael White "To protect people from the effects of folly is to
fill the world with fools." -Herbert Spencer, 1891
Stephen M ) wrote on Thursday 17 March 2005
08:01 am:
Michael,
I can think of a couple of reasons for imperfections:
* Bubbles
* Dust particles (either airborn or reintroduced to the can)
* Cured chunks of poly contaminating the can.
How to mitigate the problems up front:
Bubbles: Thin the poly a bit (maybe 4:1 poly to thinner ratio) and watch
your application technique, as others have already suggested.
Airborn dust. Vac the area before finishing (but not right before, let the
kicked up stuff settle) wipe down your work with a tack rag. Thinning will
help with this as well. A thinner layer (created by the application of
less viscouspoly) will create a smaller "dimple" when the dust lands on
your
work. Personally I thing full strength poly is only for "quick 'n thick"
protection, but appearance does not matter finishing.
Contamination: Always pour into a separate container and work from that.
Never pour back the extra. I find the yogurt cups work very nicely for
this... they come with lids too.
Chunks... It's time to buy a new can of poly.
How to recover from the imperfections that you have (there will always be
some unless you finish in an IBM silicon wafer production facility):
A cabinet scraper is your friend. This will knock off the high spots and
not remove any other finnish. If you don't have one, scrape the surface
lightly with a utility knife blace (just the blade, not in the knife) with
the blade scquare to the surface, and moving in the direction of the face
of the blade. You want to scrape, not cut. A light touch will knock off
the dust nibs .
Without the high spots, your between coats sanding will only lightly
abraid the surface. You should not be trying to level the finish. Use a
block.
Cheers,
Steve
"Michael White" wrote in message
link.net...
Hi all,
I'm trying to put a polyurethane coat onto a bookcase I'm building.
Since
it's a prototype, it's made out of cheap, soft pine. I've got it
stained,
and I'm trying to put on the final coat of polyurethane. But I keep
getting small bumps about 1 mm in diameter in the polyurethane coat. If
I sand them down and put on another coat, they come back (although I'm
not sure if they're in the same spot).
This seems to occur no matter how thick or thin I put the polyurethane
on. When I apply it, I've tried both not dragging the brush across the
rim of
the can and just knocking off the excess. I wipe down the surface after
sanding.
I think I've got about five or six coats of varying thickness trying to
get
rid of these bumps. What the heck am I doing wrong?
Thanks in advance.
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