Simple oil-based paint question which the Internet can't answer (easily)

[MRodent]

I'm asking this question because I've literally spent a day trying to find the answer ... but have become totally baffled.

I'm repainting some woodwork in a 1930s house, starting with the casement window frames and casement sashes. I've taken the decision to use OIL-BASED paint, so that should hopefully narrow down the possible answers: for the sake of my mental serenity I'd much prefer it if no water-based solution were even alluded to!

I've no idea whether this woodwork has in fact ever been repainted over the past 90 years. There is **some** bare wood, though mostly the paint seems very nice and thick and protective. Aside from a couple of window lintels (is that the right exression?) which are rotten, and which I intend to replace, all the rest of the wood is sound sound sound.

So my very simple question is: having applied an oil-based primer, do I then need to apply an (oil-based) undercoat before applying an (oil-based) topcoat (or maybe 2)?

Obviously this applies only to the places where there is actually bare wood. There are lots of videos about painting on top of existing paintwork, of differing states of wear/deterioration.

I have searched long and hard to understand whether an undercoat between the primer and the topcoat is or is not needed (in an OIL-based paint context). Please give me the benefit of your wisdom.

PS implicit in my question is: what exactly is the function of undercoat in this (oil-based) context?

 

[opps]

I am an old skool decorator. I prefer oil based paints. I like the way they flow and lay off. Additionally, they have greater longevity.

I do however use Leyland Trade acrylic primer on bare timber. I like the fact that it sands back without clogging my sandpaper.

You have gone for oil based primer, nothing wrong with that though. Ideally you would spot undercoat the previously bare bits of timber and then apply another full coat of oil based undercoat over the whole lot. Then the gloss. You can apply a second coat of gloss if you want to, but don't wait longer than a couple of weeks. And if you do, you will need to abrade the gloss with 240 grit silicone carbide.

You don't need to apply a second coat of gloss. I often do because it gives it a "fuller finish".

At this time of the year, the gloss may drag on hot days. Owatrol oil will help it flow and maintain a wet edge.

Not cheap but a brilliant additive. You only need to pour a tiny amount into the tin.

If you want to accelerate the curing time, you can add a tiny bit of terebene. It is an oxidising agent, often blue, but it will not change the colour of the paint

 

[Wayners]

Read the data sheet for the type of paint you are using.

Example.
Leyland undercoat.

Data sheet says.

Type of substrate.....

Wood and metal.

Means it can go over wood and metal.

If you read the application data sheet online is says to use the appropriate primer. This means if it's stained use a stain block. If it's plaster or other dry surface use something like Gardz.
If the paint is peeling use peelstop.
You get the idea.

There are universal primer undercoats like zinsser coverstain but you need to follow the data sheet.

You can use a standard oil based undercoat if all you are doing is wood and old gloss

 

[MRodent]

I am an old skool decorator. I prefer oil based paints. I like the way they flow and lay off. Additionally, they have greater longevity.

That endorsement helps.

If you want to accelerate the curing time, you can add a tiny bit of terebene. It is an oxidising agent, often blue, but it will not change the colour of the paint

Interesting: because I will inevitably be painting into September, October ... and it appears that oil-based paints can become slower to dry in colder weather: not a problem for the sashes, which can be dried indoors, but the fixed frames will be trickier: Rustins seems to be the most common terebene product. Expensive, but you say only use "a tiny bit". Would you use it on all three i.e. primer, undercoat and gloss topcoat? Currently I don't have a clue how fast these all dry relative to each other ...

 

[opps]

That endorsement helps.


Interesting: because I will inevitably be painting into September, October ... and it appears that oil-based paints can become slower to dry in colder weather: not a problem for the sashes, which can be dried indoors, but the fixed frames will be trickier: Rustins seems to be the most common terebene product. Expensive, but you say only use "a tiny bit". Would you use it on all three i.e. primer, undercoat and gloss topcoat? Currently I don't have a clue how fast these all dry relative to each other ...

Terebene is cheap. I buy the Rustins stuff from my local Leyland SDM.

Owatrol oil is expensive but a god send for oil based gloss when extremely hot or cold. When hot it is difficult to maintain a wet edge. When very cold the gloss drags as you try to apply it. Owatrol helps the paint flow in both cases.

I haven't used oil based primer for years. I tend to prime with water based UC/primer (Leyland Trade). It is the only waterbased primer that I find doesn't clog my abrasives. It is crap at colour obliteration though. That said, I expect the oil based undercoat to do that (obliterate the colour).

Late September and October (in London) I would only use the Terebene on exterior gloss if I were concerned about rain.

I have no idea which paints you are using. My go to for exterior woodwork is Dulux Trade (oil based) Weathershield. The undercoat is really well behaved. At this time of the year it is (slightly) touch dry after a couple of hours. It has a slightly higher sheen level than regular undercoats, that means it is easier to keep clean prior to applying the gloss. If you want an oil based satin finish, look at Johnstones Storm Guard.

The gloss however will end up with dimples in the event of rain. The extent of cratering is a function of both the time since applied and the intensity of rainfall. Annoyingly, the ridges of the tiny craters attract dirt. Adding Terebene means that after about 3 days, you can flatten them with 180-240 grit silicon carbide paper. Without the terebene, after 3 days, you risk ripping the surface of the paint.

Occasionally, I have to do exterior jobs over winter (not ideal). I will add terebene to all oil based paints.... getting back to your job, if you are using exterior gloss, try to apply it early in the day. As winter approaches, the evening dew can make wet gloss paint less shiny.

I hope the above makes sense, if not, post back.

Oh, and buy a BrushMate 4 container. It keeps your oil based brushes wet when not in use. It uses vapours to keep them wet. They now seem to be about £28, but at the end of each day, you just hang the brush inside. Caveat warning- My local Leyland SDM only sells the crap version. They removed the cellulose thinners type ingredient (MEKO) from the vapour pad to appease the EU. You can however now buy the original MEKO formula again in bottles and pour it onto the pad. I leave brushes in the boxes for years and wet the vapour pad using the bottle.

 

[MRodent]

Thanks. These are the paints I've bought so far.

Primer: Leyland Trade Wood Primer
Undercoat: Leyland Trade Undercoat (oil-based)
Topcoat: Dulux Trade Weathershield Exterior High Gloss White (glad to see you give Weathershield your thumbs up)

I had to take out a mortgage for the latter: £55 (cheapest found) for a 5L can from Amazon. I'm committed now! Buying smaller cans you obviously pay more per litre.

The latter 2 haven't yet been delivered ... and the undercoat could therefore be returned without hassle (Amazon), i.e. in favour of Dulux Weathershield oil-based undercoat. The latter is currently retailing at Amazon for, er ... £94 for 5L. Much as I want to do a good job ... What's your view of Leyland Trade Undercoat?

BrushMate looks great. £21 at Amazon, probably the crap variety. ... MEKO bottles... OK! Life ain't so simple these days.

Owatrol ... if things extend into colder weather (which I suspect they will).

Gold dust all this, thanks again.

 

[opps]

I have never used DWS gloss over anything other than the DWS oil based undercoat, and to be honest, I have never wanted to. I don't want the customer phoning me and saying that the finish case failed.

£55, that must be the 2.5L tin? I paid £59 in my local decorators' merchant today for the 2.5L DWS oil based undercoat. The last OB DWS gloss 2.5L was similarly priced. 5L, I would expect to pay about £100+

£21 for the BrushMare 4, bloody good price.. buy it and then pay £8-10 for the MEKO bottle. For a Brushmate 4, that will last upto 3 or 4 years. The pad has a couple of little stars on it (stapled on). Once the supplied pad starts to dry out, the stars fade. I simply pour the MEKO onto the pad to rejuvenate the pad. From memory, a replacement pad is about £7. The bottle is wayyyy cheaper.

I knew I was being mugged off when I paid, not £28, but £38... but I needed it to be on site (Leyland SDM)... I will pour some MEKO onto the pad tomorrow.

I will however check out Amazon for more BrushMate containers. Thanks