Painting - difference b/w paint & primer

I am painting my daughter's outdoor playhouse. We have very little money, but lots of extra paint from old projects in the basement. Is there a reason not to use 2 coats of paint rather than primer and then paint? (Both coats being latex)

Alternately, is there a way to modify paint in order to use it as primer?

And in the same vein, what is the difference between interior and exterior? Is there a way to modify one to serve in place of the other?

Thanks in advance! :)

- 1954House mom

First interior paint is not formulated for the weather outside. Using interior paint on the exterior almost always ends up in paint failure.

While primer is defenitetly better [and sometimes absolutely necessary] You might get away without using it. I would recomend thinning the paint 10-15% and also adding 10-15% emulsa bond to the first coat of paint. Make sure both coats are exterior grade paint.

Primer help paint adhere

you can increase your chances of success whatever you do by making sure the surface you are going to paint is clean and free of any loose paint. any surface contamination is going to reduce the bond. Is this wood you're painting? previous paint? careful use of a pressure washer might be warranted. clean, clean, clean!:D