Bitumastic versus Comastic

[Akura]

How do I know what was used last time my hull was blacked, and does it
matter whether I use a bitumen based or vinyl based coating on top of
whichever was used last time?

[Mike H]

Post by Akura
How do I know what was used last time my hull was blacked, and does it
matter whether I use a bitumen based or vinyl based coating on top of
whichever was used last time?

Not sure how you tell what already on it, but the as I understand it you
can put bitumen on top of comastic, but don't put comastic on top of
bitumen. This is what a marina did to our boat a few years ago, the
result was that it just bubbled up and fell off. Last time we did it
ourselves using a bitumen based blacking, 12 months on still looks good.



Mike H

[Tim Leech]

Post by Akura
How do I know what was used last time my hull was blacked, and does it
matter whether I use a bitumen based or vinyl based coating on top of
whichever was used last time?

It does matter, if possible stick in general terms to what's already
on, ie tar/pitch based (includes Comastic & Coflex) or bitumen based.
Note that Bitumastic is a brand name, they produce both types of
coating.
It's fairly successful putting bitumen onto well weathered tar, Rytex
is supposed to be particularly good for this. Tar onto bitumen is less
successful, but again not so bad if the bitumen has been there for
several years. You're likely to find the 'broken' edges of the old
coating curl up slightly.
Tar coatings are generally more durable, but there are increasing
environmental and H&S concerns over its use, so it's likely to become
harder to get hold of. For that reason alone I wouldn't really
recommend moving in that direction.

Cheers
Tim

Dutton Dry-Dock
Traditional & Modern canal craft repairs
Vintage diesel engine service