Water-Based Finishes for a Straw Board Product

Advice on how to seal before staining, to minimize swelling and grain raise. October 2, 2005

I was asked to do a staining sample for a new project. It is strawboard WB stain and WB clear coat, which is an environmental challenge here in BC, Canada. It swelled the second the water hit it. I'm looking for ideas from anyone who has worked with straw board.

Forum Responses
(Finishing Forum)
I've seen strawboard, but have never finished it. Obviously, one solution would be to seal it with something not waterborne, like shellac, and explain to the client that the tradeoff of using one thin seal coat in the entire finishing process that is solvent borne is worth it (still mainly waterborne). You could point out that even your waterborne coatings still list some percentage of VOCs.

Another method would be to minimize the amount of grain raise/swelling by shooting the first coat or two very lightly. You probably can't get a film to form at less than 1 wet mil, so you will actually have part of the surface unsealed. Perhaps you could try 2 or 3 dry coats with a really light scuff (320 or 400?) between until you get the whole surface sealed.

After/if you seal it, start building with another coat or two before you give it a good leveling sand. You want to have the swollen fibers protected with enough resin so that when you sand, you don't cut through to them. You may have a rough finish for the first 4 or 5 dry mils, but at that point, you can probably give it an aggressive sanding and proceed normally with more finish sanding and topcoating.

I agree on the shellac base. You could use Zinsser seal coat first to seal the wood and then spray your topcoat over it. Be sure to check for compatibility first, though, just in case.

Another great option is to use a wb sanding sealer first. I have used Van Technologies WB products for some time and their 240 sealer, which seals the wood and has a high build without swelling it, followed by the 480 or 482 topcoat (a good combo). They also have a urethane sealer that you could try. It is more expensive, but clearer and in my opinion, better.

If you use the Vantech wash coat before staining, you can also greatly reduce the swelling of the wood.

From the original questioner:
Thanks for all the help. The sample looked great and was pretty simple. First I sealed all the end grain and then stained, then let the stain dry well before spraying two coats of sealer, and sanded at 320g. Then applied two clear coats. The customer was very happy and we got the job. The large job is a different story than small samples. I might have to rethink when doing inside corners.

Here are two other options you may want to try. Use any one of the drying oils, wipe on, and allow to dwell for 5 to 7 minutes, then wipe off and allow to dry (you may need two applications). This will serve as your sealer. Once the drying oil cures, it will become a reactive coating, and will not be affected by the WB coating. You may also want to consider spraying a toner for color instead of staining.