Inevitable springtime varnish questions!

[Kelpie]

I'm a complete newbie when it comes to varnishing, and am attempting to get my composite (i.e. marine ply decked) Wayfarer back in the water. By the end of the this week!
I've stripped and sanded as much I can be bothered to, and did a few coats of varnish on the hatch cover as an experiment, which has led me here, cos it didn't go quite as well as I'd hoped.
I'm using Blakes Duragloss, and on the hatch I thinned it 20% with no.1 thinners before applying. Incidentally the thinners was International, not Blakes, cos that's all that they had in the shop.
After leaving it for about eight hours I put on another coat, and a third one after a similar length of time. According ot the instrstructions on the back this would be fine and would save me sanding in between coats.
Oh and I also shook and stirred the can, which I've just read is rather a foolish thing to do (oops).

So, the hatch turned out with a fairly rough (over-brushed?) and bubbly surface, which I'm not very happy with. I left it a few days to thoroughly dry and then gave it a firm sanding all over with fine paper, before applying an unthinned coat. It's still working out a bit bubbly though.

I'm sure I've made more than one mistake, but would appreciate any advice on what I can do differently before starting to varnish the deck itself. I'm not looking for ultimate flawless finish, just something that looks good and servicable, and as I said I'm keen to get back the water in just a few days time.
Not much to ask, is it!

 

[Salty John]

The bubbling could be due to moisture in the wood before you started, it is certainly caused by shaking and stirring the varnish and it's also a result of bad drying conditions. Bad drying conditions combined with too much solvent is particularly bad.
I'd use varnish unthinned except on bare wood in which case I'd thin 10%.
Don't varnish when it's cold and wet - I think this means you can only varnish in Scotland on the second week in August.
Leave at least overnight between coats - I think you didn't leave enough time between coats when, it seems, you had bad drying conditions.
After you've sanded be meticulous about wiping down - if you leave sawdust behind it will rot and turn black and also leave a rough surface.
Hope this helps.

 

[Pasarell]

I agree with all Saltyjohn has said but add the following.
Unless you strip to bare wood there is no benefit in thinning the varnish to the extent you did. If it's cold about 5% thinner will aid flow but no more. (Incidentally both International and Blakes no 1 is white spirit so far cheaper to buy as such).
There could be various reasons for your bubbles but I suggest it is solvent entrapment. Even if you hadn't thinned to the extent you did you should have left longer between coats unless indoors and heated. Check the tin for overcoating times.
If you don't rub down to remove all the bubbles the varnish will not harden through and will be soft all year.

 

[pmyatt]

More coats of varnish is the answer, total of 6-10 coats depending on where you are - there are quick varnish jobs and good varnish jobs - there is no such thing as a quick good varnish job - and anyone who says otherwise is talking b******s. It ain't called bright"work" for nothing!

 

[Kelpie]

Thanks for your replies
FYI, the 'drying conditions' for the hatch cover which I experimented on were my living room! So I reckoned I could follow the instructions on the tin as per 20 degrees C. I quote:

"Recoat at 20C- minimum- 4hrs; maximum- 48hrs
Recoat at 10C- minimum- 8hrs; maximum- 4 days

If maximum recoat time is exceeded, rub down"



The boat itself is in a garage, which is dry if not as warm as the house. Point taken about varnishing outdoors in Scotland- I don't envy my friends working on their yachts at the moment, especially those doing epoxy work...