Painting - Oil Primer Running

I have a new set of oak stairs. I've stained and top-coated the treads. The risers and stringers I wanted to paint white.

My problem is that the oil primer is 'running' onto the treads whenever I try to prime close to it -- which gives a very uneven look. It almost looks like it's being 'sucked' into the crack between the stringer and the tread.

I've tried tape, but it just makes it worse. Any suggestions?

You are likely applying the primer too heavy. Two thin coats may work better. If you take a putty knife with a rag wrapped over the blade it may be a little easier to remove any paint off the treads. Make sure you are using a quality bristle brush, sized for the application. An angle brush may be helpful.

I think I may have done the painting first,too late for that though. You are,as marksr said, putting it on too heavy. This is a case where I would use tape,given it is already stained.. If you use the blue stuff and burnish it down[not so much as to damage the tread :wall: ] there is no reason That paint should get under it. I don't think the tape was being put down right.You need to do more than just lay it down.

Another point is to not start to paint where the stringer meets the tread----end there with the drier brush.