Wednesday, January 7, 2015

Gleaming polyurethane

Well now.  Here's a nice looking plywood floor, if I've ever seen one:

I hadn't used a polyurethane finish before, so I decided to try it on the floor first.  It'll be covered by the mattress most of the time, but it still needs to be durable and waterproof.  Any screw-ups (as it turns out, it's pretty easy to apply) will be hidden, and it's great practice for the interior and the galley.

Here are the steps I've recently learnt to apply a nice, simple poly finish, at least to softer stuff like this Birch.  It seems like a lot of steps, but it's really just 3 coats of decent oil poly, sanding and clearing off dust between coats:
  1. Don't buy rub-on poly.  I used that for the cabinet knobs.  It makes a nice finish in the end, but it's thinned 50% to make it "rub-able" so you need about 6 coats, which is a huge hassle.
  2. Sand, first with 80, then 220
  3. Quick knap with 220
  4. Brush off and let the dust settle, literally, overnight
  5. Rub with a cloth or suchlike to clear dust, then again moistened with mineral spirits or thinner (NOT water, water will raise the grain again and won't play well with the poly)
  6. Apply coat 1, let dry 24 hours
  7. Light sanding with 220 to get rid of raised grain and dust nibs
  8. Cloth rub, again with the paint thinner
  9. Apply coat 2, admiring how it's starting to coat the wood instead of soak in, dry 24 hours again
  10. More sanding with 220 ... starting to feel tedious now
  11. Cloth rub, thinner, wishing it were done already
  12. Apply coat 3!  If it's done right, this will be the last one, wait for 24 hours to be sure ... make sure to move slow and go lightly over the last brush strokes to pop the tiny bubbles that form in the poly (If bubbles were an issue in the first 2 coats, thin 10-15%)
  13. Pat self on back for doing such a nice looking finish
  14. Sand very lightly with 2000 polishing grade, just enough to get rid of the few remaining dust nibs ... too much will take away the satin sheen and make it glossy

Clearing dust is the hard part.  There are always a few little nibs.  I think I did as well as I could have without making a dedicated finishing area, and that would be GETTING CARRIED AWAY, which I promised myself I would not do, in my very first post.

This is the Minwax floor-grade satin poly, which I'll use throughout the interior and galley.  It sure soaked into the soft birch on the first coat, and even a little of the second.  But by the third, it came out nicely levelled and pretty well free of bubbles.  I went with oil instead of water based because it's supposed to be a little more durable.  Who really knows.

The T-nuts stay exposed, but they're evenly spaced and more or less flush, so that's fine.

Back to the walls!  This needs to cure for awhile, which will result in some awkward sawhorse setups, but after that I'll cover it with cardboard and work right on the trailer.  I've decided to build them lying down, finish the interior, then stand them up.  I think the only reason I wanted to assemble them standing was to make progress right away, but ...

... patience, young man.  Patience.

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