Primer/undercoat properties

[slawosz]

Hi,
I will soon be using primer/undercoat on my boat. I never used one before, and not sure what I should expect. After reading about priming, I understand that primer will have relatively thick layer that will help smoothen the surface - is it the correct assumption?

 

[anoccasionalyachtsman]

I've done a fair bit of painting all sorts of stuff but I'm not the chemist that you possibly need..

Primers come in various flavours with three separate purposes. The primary function is to get a bond with the material you're painting. It could be an etch primer for metal, a solvent type which gets into the surface of a plastic, or something that's good at soaking into wood. To get as close to a mechanical grip into the surface a possible. The second is to be a barrier between a base material that might react to the undercoat or topcoat and give a horrible finish. Lastly there are filler primers, and their job is the one you've picked up on. They help fill minor imperfections and are often applied in contrasting colours and sanded between coats. Not needed on a good base, and in my experience are quite soft and also not particularly waterproof so I'd hesitate to use a cheap one underwater.

Undercoats are used to build a film thickness with something that's often a bit cheaper than the topcoat, and help to cover contrasting colours. They too need to create a bond with the surface beneath. They can also act as a barrier between layers that would otherwise react.

All the above gives clues as to why it's better to stay with one manufacturer and stick rigidly to their directions!

 

[Poignard]

Paint manufacturers such as International and Epifanes produce very good guides in booklet form that would probably answer any questions you might have.