I have a garage where the interior is OSB. Can I use an exterior paint on that to avoid having to prime it, or will I run into bleeding problems?
Hey there montanapainter,
You're right in your assumption on the exterior paint. That's what I've found always holds up the best. But I wouldn't recommend skipping the prime step.
OSB is constructed with water based glue and ink, and also can contain various amounts of sap. By not priming, you'll find that the sap will bleed through your paint even through 2 coats, and that's not what we want to see.
Can you use a latex based primer if you already have it? Sure... But the reason I recommend that you use oil is because like I mentioned above, the OSB is made with water based glue. By nature, Oil and water do not mix, so it's good for sealing in those glues and sap that would otherwise bleed through the paint.
I've seen it achieved with both oil and latex, but I prefer the oil. Give it a coat or two of the primer and you should be good to go!
And remember that the Oil primer will \\\ smell ///, so be sure to keep a window open while you're using it. Also, it's tough to get a drywall smooth finish on the OSB, so don't worry if a bit of the texture still shows through.
Good luck with it, and be sure to keep us posted on how it goes or let us know if you have any other questions!~
BIN is NOT an oil based primer. It is shellac with white pigment in it. BIN is an very good vapor barrier and is not recommended for exterior surfaces because it will not allow vapor to respire outward, i.e. expect peeling. The party wall to an unheated garage would be considered an exterior wall. There is a big debate in the construction industry as to whether walls should have an absolute vapor barrier on EITHER side, especially in humid areas of the country where A/C is used commonly.
You can use either water based or oil based primer on the OSB or MDF. One downside to the water based primers is that they will swell the OSB somewhat, resulting in a rougher finish. Also, water based primers do not sand well, as do oil based.
I always treated garages as if they were interior surfaces, mainly because they are not subjected to rain of direct sun. Exterior paints are generally made more elastic to cope with the extreme temperatures and rain/humidity found outside. I don't particularly see a problem using the exterior in the garage if he already has the product and wants to use it up, however. My preference would be for interior paint, however.
Ah, many thanks for catching my mistake. You are correct, it is indeed a shellac-base primer. I've edited my previous post to avoid incorrect information on the product. I must have missed that when I was putting in the other Zinser product and simply overlooked it. Thanks!
Glad to have a pro in our midst! = ) Much appreciated.