You're quite right of course Scooby, however I'd assumed from Magnus's question that he would be sanding the boat back fully before repainting it as he was asking about a change of colour....

Whenever I've had a painted boat (as now) I've always maintained it in that colour by rubbing it back every couple of years and then giving it one new topcoat - ergo, no weight gain (in theory)

On the one occasion I DID decide to change colour (went from an orange Swift to white), I sanded the boat right back almost to bare wood before applying two coats of white undercoat and finally one liberal white two pack topcoat. Again I don't think the boat gained any weight and may have even lost some!

The exception to this must of course be someone painting a gelcoated boat for the first time. As said in other threads on this site, it really must be a last resort as there are other ways to revitalise dull gelcoat...

I still think that Silver looks really cool, the only downside is that metallic paints are slightly heavier than plain colours and I'm told that some paint colours are heavier than others anyway. I'm assuming that clear will be the lightest, but does anyone know if there is a source anywhere on which colours are lightest (in weight), assuming the same covering ability?