Topcoating exterior wood with Bedec MSP, primer or no primer on some now exposed wood?

[DaveyH]

Hi,

I was originally going to use Bedec Barn Paint, but I've read that their MSP stuff has more resins in it than their barn paint, making it a slightly better paint apparently. I've also read good things about Sadolin Superdec, but that's a bit harder for me as a DIYer to get hold of. However if Sadolin Superdec is in a different league to the Bedec MSP I will make the effort and get some, but if it's more or less on par with the bedec MSP then I'll just stick with Bedec MSP?

Anyway, while doing the prep with some 120 grit paper. I've found that some of the old flakey paint has sanded back to bare wood in places. Here's a pic.

pic 1b.jpg (128.06 KiB) Viewed 894 times

Now my main question, would it be better to go over it all with a primer like Zinnser 123, then 2 or 3 coats of the bedec MSP or just do 2 or 3 coats of bedec MSP?

I mainly thinking about what will last longest?

Thanks in advanced.

[dynamod]

Superdec is my go to outside paint. I'd spot prime bare wood with 123 and give 2 or 3 topcoats. Follow the re-coat times though. For all it can be touch dry in a hour or two, the full 16 hours benefits application.

Barn Paint is OK, if not brilliant. Superdec every day.

[DaveyH]

Superdec is my go to outside paint. I'd spot prime bare wood with 123 and give 2 or 3 topcoats. Follow the re-coat times though. For all it can be touch dry in a hour or two, the full 16 hours benefits application.

Barn Paint is OK, if not brilliant. Superdec every day.

Thanks for replying.

I will get some Sadolin Superdec then. It might actually work out a little bit cheaper than using the Bedec MSP because it's available in 250ml tins, whereas I would have to buy a 750ml tin of Bedec MSP. One 250ml might even be enough because I'm only painting about about 1 square meter.

Is there any particular scientific (or best practice) reasoning behind why you would only spot prime the bare wood bits and not prime over the whole lot, or is it really just about saving a bit of time, paint and money?

[dynamod]

Spot priming is belt and braces I admit, but as Superdec is self undercoating and grips well, a full primer coat isn't necessary.

[DaveyH]

Spot priming is belt and braces I admit, but as Superdec is self undercoating and grips well, a full primer coat isn't necessary.

I've been picking at the old paint this morning with a small scrapper blade and sandpaper and I've managed to get most of the area back to bare wood now, so I'm just going to prime the lot with zinsser 123 then 2 or 3 coats of superdec. I just need to decide whether to go for Gloss or Satin.

Strangely, based solely on looking at online pictures of the tins, the Sadolin Superdec Gloss only claims 8 years protection, whereas the Satin claims 10 years protection.

[dynamod]

I'd go with the satin, as a lot of these waterborne exterior gloss finishes, don't have the expected sheen level of traditional (oil) gloss finishes. That said, satin (being a lower sheen) will reflect less light back and prevent every scratch and ding in the wood being seen. In addition, it just looks better, as the semi-gloss of the higher sheen finish looks a bit naff IMO.

Try to avoid applying this stuff in direct sun, as it dries very rapidly under these conditions.

[DaveyH]

I'd go with the satin, as a lot of these waterborne exterior gloss finishes, don't have the expected sheen level of traditional (oil) gloss finishes. That said, satin (being a lower sheen) will reflect less light back and prevent every scratch and ding in the wood being seen. In addition, it just looks better, as the semi-gloss of the higher sheen finish looks a bit naff IMO.

Try to avoid applying this stuff in direct sun, as it dries very rapidly under these conditions.

Yeah, I will go with the satin, I much prefer it over gloss as well.

The sun goes around the back of the house after about 2 o'clock, so I can easily avoid the directly sunlight issue. .Will it be a good idea to add a small splash or 2 of Floetrol in the superdec in this hot weather or should it flow nicely without it?

[dynamod]

Floetrol should work fine. Even a small dash of water doesn't hurt, though over thinning needs avoided.

[aeromech3]

Made a mistake, years back of, using a microporous paint ( barn type) on my wood external window ledge; it is designed to let the wood breath or summit, lasted two years and started to bubble.

[dynamod]

Those microporous paints were an oddity. Over bare wood they seemed to perform quite well, but over an existing paint I never really saw the point.

It's like wearing a bin bag under a Goretex outer shell. What's the point of that, as you're going to get soaked from the base inwards.

[sammy.se]

I painted a home-made pine trellis with Bedec MSP, without Primer, 5 years ago, and it is immaculate to this day. Very impressive. This was on fresh, bare wood.
I also painted my uPVC window (externally) with the same Bedec MSP and also great after 5 years.

If I was painting over old paint, I would probably prime using Zinser BIN or 123, for peace of mind, before applying any top coat. That's just me.

[dynamod]

would probably prime using Zinser BIN or 123

123 will be fine, but don't coat large areas with BIN in an exterior setting. Over knots and some resinous patches, fine (though I'd suggest aluminium primer in certain situations) It's far too hard and inflexible to be used successfully outdoors, as the wood expands and contracts way more than BIN is able to allow for.

Old paint is best given thorough preparation (scraping, sanding, filling) Zinsser primers are perceived in a strange way by many, and seen as a substitute for correct procedure. Whilst they are useful, and do solve certain problems; they are an addition to, not a substitute for, correct prep.