I'm currently trying to repaint my barn on pressure washing off the paint on the outside it was originally done with latex paint. I'm wondering how complete of a job I need to do to get the paint off or if the oil base paint I'm going to put on will penetrate through it.

 

Post a picture so we can see what your seeing.
Old based should never be applied over Latex.

 

What type of siding is under the paint? Why are you considering going with an oil based paint now? ...because of the problems you are having with the previous paint? Is the base coat the original paint? How large of an area?
I used to be a contractor...but it was many years ago...take my advice with a grain of salt.

Generally speaking, you would want to apply a transition or barrier coat to get the oil based paint to stick....but there are other problems. The latex paint will expand and contract a lot more readily than the alkyd or oil based paint will. If you apply the oil based paint on top, you may have issues down the road with that....but the big thing will be getting it to stick. That is what the barrier coat will do. You will have to check with the paint manufacturer for a recommended product or product type. I don't know of any offhand because of the oil based topcoat.

The other thing is, if this is wood or wood based siding, if you are having a problem with the paint that is already on there, then you would either want to remove ALL of the existing paint, and start over, or attempt to seal the stuff that is leftover so that it does not come off too easily in the future. If it was concrete, or something else that wouldn't be damaged by the pressure washing,and you could easily remove ALL of the existing old paint...that would probably be the best bet. Since it is a barn, I'm assuming wood or wood siding.
First, depending on the surface...you will probably do a lot of damage to wood or wood based siding products before you get all of the paint off with a pressure washer. If it is coming off easily...then maybe you should carefully continue, and try and remove everything that will come off without damaging the wood.
At this point, I would still recommend a latex paint. Either way, you will need a good primer. If it is rough wood, you will need to brush, roll or at least "backbrush" or "backroll" the primer to work it into the voids and pores of the wood. If you rough up siding and have exposed the material underneath the finished surface, you will also need to seal that...but it will never be as good as the finished surface was.
You could use a primer or shellac based primer like Zinsser, but I don't know if anyone is still making something that would be good for sealing wood in for an exterior oil based product topcoat. The problem will be that in the future, water will get under the edges of the existing paint that you are unable to remove. Usually, a painter will use Zinsser as a spot primer, and apply it with a brush or roller to work the primer/sealer into the edges and open pores surrounding the remaining paint. That will be the key to success there....working the primer/sealer into the edges and pores around the remaining paint to prevent moisture from getting in behind it. This isn't usually recommended by the manufacturer for large areas...just spots. It works pretty well for a latex paint topcoat though.
If there is a lot of bare wood that is exposed after removing the old paint, you may be able to spot prime the edges of the remaining paint, and then use a wood primer that is recommended by the paint manufacturer to prime the rest of the wood. Just make sure that you check to see if the primer is compatible with the shellac based (or whatever you decide to use) spot primer/sealer first.
Unless you are painting metal, latex is usually going to offer the best adhesion, and ability to withstand temperature changes while retaining it's color far better than oil. Probably even twice as long.

Sounds like a lot of work! I don't envy you.

I hope this helps...but keep in mind that I have been out of the industry for a couple decades. There is probably (definitely) someone out there that knows much more than I about current products.

Bullseyeguy (new guy on the forum)

 

There is a base coat of what I'm assuming is oil base that does not come easily. And a latex on top of that. I'm going oil base so it will last longer. It's wood tongue and groove. I can actually get the second coat off of the first in some of it. But that first coat will not come off

 

the paint that is still there will not come off without peeling wood. i am planning on using an airless paint sprayer to put the new paint on

 

Isaac,

I would use an oil based primer, but you really need to backroll it to work it into the voids. I would also backroll the finish coat(s). If you just spray it, you will have the same problem again in a short time. The backrolling will be an important step.
I would still recommend latex paint...it will also be a lot easier to work with. I would also recommend TWO finish coats...even if you tint the primer. You need something to get a layer of protection built up on top. The primer does NOT do that...it will just make the paint stick to the wood.

Unfortunately, that siding is always going to give you trouble...but you've done a lot of work so far. Might as well finish it as best as you can. Looks like you've done a good job.

Good luck!

 

I'd use a solid stain rather than paint. Latex usually doesn't fade as quick as oil base.