I'd do a shellac based exterior grade primer. And then a coat of the paint. On all sides of the uninstalled boards. Then do a second coat on the installed surface. A good primer's job is to seal and to provide grip for the topcoat. A topcoat simply doesn't have the grip needed to stick long term to wood exposed to exterior conditions without the wood being primed first.

You can put latex over oil But not oil over latex. What NEEDS to be prime is bare wood AFTER it's been sanded. What needs to be sanded is any wood that is greyish in color or has checking. Primer sticks best to clean wood. Paint sticks best to a prepainted surface, hence the use of primer or the practice of painting over paint. IMHO spraying paint is a huge waste of time and paint. In no way does it matches the kind of job or quality you get from hand painting.

The siding was installed in November and was caulked at that time. Unfortunately the temperatures dropped from 70 to 30 overnight and didn't warm up again until April. We live in Iowa so the siding has seen its share of rain & snow and now finally warm sunlight. It seems to look the same as the day it was installed other than being a little muddy near the foundation. Every painter that has been out to look at it has said that they will clean it well and apply two coats of paint. The only major discrepancy between the bids is whether they prime the whole thing or just spot prime the areas that are caulked or cut plus the cedar porch rails. Only one painter has wanted to prime the whole thing and his bid is $2100 more than the next guy. Money is definately tight right now as we just finished building and are still in the process of closing on our mortgage. I'd love to save the $2100 but then again its not worth it if it results in an inferior paint job and we end up having to redo it in a few years. Thanks for any advice you can give me.

I have several questions. First, are the nails galvanized or stainless steel? If they are galvanized, I would prime them first with Ospho before caulking and then prime with a regular primer. Ospho is a green liquid that will turn rust black and prevent bleed through on your paint. Galvanized nails rust fairly quickly and this will head off a problem in a year or so down the road. You can also use Hammerite Rust Cap Primer for the nails. (Many painters tell me that QTips work great for applying Ospho). If they are stainless, I would caulk first and then prime the surface. This will prevent any flashing effect from the Ospho on the finish coat. I would actually prefer to use a spackling compound that will stay somewhat flexible, but you can use caulk also. Is the siding a hardi-panel or hardi-plank? I have not seen any real wood siding pre-primed in my market. If it is hardi-plank, I would prime the siding with a good latex 100% acrylic primer. You could use 123 by Zinsser, Suprime by Pratt and Lambert, or FreshStart by Ben Moore and then topcoat with your choice of paint. Every paint company will manufacture a 100% acrylic primer to use with their topcoat. The primer used on siding as well as steel doors is to protect the product in shipment. It is still intended to be primed before painting. I would highly recommend that your primer be manufactured by the same company as the paint for warranty reasons. Make sure you use a really high quality caulk. I have found that Big Stretch Caulk by Sashco is the easiest to work with of all the caulks on the market. It is paintable. It is a little expensive, but is very flexible and smooths out great. Consider having your primer tinted 1/4 to 1/2 formula of your topcoat color. It makes the paint cover better. The primer will still be a little light to the finish coat so you can see any areas you have missed when applying your topcoat. I do recommend two coats of finish paint.