I'm making an exterior sign approximately 4x6, and am need of some helpful advice on the following. (I have wisconsin weather)
1. MDO board?
2. What primer or paint is best? I would like to paint it black.
3. What adhesive should I attach my wooden letters with?
4. What should I put over the to of it to seal it before I nail it up?
Welcome, I know your looking for answers for a problem but here's the deal. This is a woodworking forum and it is usually nice if people start with an intro. After reading you question I'm not sure you will be around after you get your answer and since I use to make signs. Here's my advice.
Unless its new MDO is always an exterior plywood with a medium density overlay finish. Its either good one side or both and can be bought primed already. It is used for high way signs during construction as well as many shop signs.
Any exterior primer is good. Its mostly because that papered finish can really soak up paint. Get the edges really good but i have pulled signs out of water after days of getting soaked and no damage was apparent.
Any exterior paint will do but with dark colors especially you want a premixed color rather than having a color mixed. I would also suggest an industrial paint Pit-tech is a good one but a good exterior house paint such as Behr or other brand that has a premixed black will do.
Liquid nail construction adhesive works well for placing letters on signs. You didn't say what the letters will be made of. Wood will last longest, foam and HD Foam letters will get torn up by birds and weather. Plastic letters can be applied with the same adhesive as mentioned. Titebond II or III would not be on my list to recommend.
You can paint the while board first then apply the letters for a better look. This can sometimes create a problem with the adhesive because of the paint if its not cured good before applying letters.
