I agree with MeepBeep, Thompson's waterseal is not going to last more than 12 months. I have even applied it with an airless spray painting rig and while that does get it deeper into the wood (solid boards not OSB or Particle board) It simply does not last more than 12 months.
Now, I would recommend this procedure for keeping the elements out of OSB (nothing will actually keep moisture out of Particle board that is exposed to the elements of nature except 3 coats of an elastomeric roofing coating(rubber you can roll on)).
Seal the OSB with a coat of thinned shellac, this will soak in and actually seal the pores of the wood chips that make up the OSB.
Once this is dry apply a coat of primer then when that is dry apply two coats of a good quality latex exterior paint. This will keep the moisture out for 5 years, at which point it is time to apply new finish coats. I have a coop that has an exterior of 7/16 OSB that was painted as I have outlined, it is now two years old and still looks very good and weathers heavy rains quite well. I expect to have to give it a paint job in three more years.
For any treatment of a chicken coop I would give it a full week before putting birds into the fresh coop. This will allow all the solvents to evaporate and that means it will be safe for chooks to peck at, which they will end up doing at least once.
If you are wanting a one shot done coating for OSB used as siding then the Black Jack roofing product or any other elastomeric roofing coating is the way to go. You can get coatings that are pure white, can be tinted for other colors and once rolled on and dried for 24 hours, will last at least 7-10 years. They are rubberized so they shed water completely. They are patchable should something puncture the coating and they are highly reflective so the interior will not heat up as fast as it will with a regular paint product.
Totally agree with you. That's why I laid out the seal, prime, double coat paint and repaint regimen. You can't skip a step and you have to be diligent in upkeep.
I have discovered a super product that is fast becoming my new favorite for top coats. Sta-Kool 770 cool roof Elastomeric, the stuff is 73 bucks for 5 gal but when properly applied it forms a thick, water proof coating that binds so tight to new wood that you can't peel it off when you want to. It is also tintable and actually reduces roof heat by 20 degrees if you leave it pure white. It can be used on bare wood, metal, or shingles (takes three to four coats for best coverage of shingles). If you do manage to tear it, just touch it up and the seal is good as new. I am expecting it to last on walls for around 7-10 years. On Roofs it lasts 5-7 years before needing a recoating.
Thanks for the suggestions Redhawk - hope this stuff works on cardboard (not OSB) particle board on the interior coop walls. The coop will be located under a patio roof so rain will not be an issue but our climate/atmosphere will be very humid during summer so needed the best suggestion for protecting the interior walls.