Undercoat

[Marvie]

Very time limited this year to paint my boat. If I prep it for rust, and put a coat of primer then undercoat on, will it be waterproof until next year when I can tackle the painting?

[alan_fincher]

If you mean just one coat of primer and one of undercoat on to bared steel, I think you would need to be very lucky indeed for no rusting to occur in a year.

 

To some extent it may depend on choice of primer.  Many of them offer little resistance on their own, but a few do somewhat better.

Edited June 11, 2018 by alan_fincher

[dave moore]

Most primers and undercoats are porous to some extent, gloss supplying the seal. I wouldn’t consider such an approach.

[Murflynn]

it would be a total waste of time and effort. ...........  don't ask me how I know  

[Dr Bob]

Get at least one coat of gloss on. Best idea if limited time is to do a smaller area. If you rush there's more chance of a botched job. Please watch the weather and particularly temperature and dew point. Weather delays are inevitable in the UK so plan for them.