A Guide to Undercoat and Paint Primer - Decorator's forum UK (2024)

I’ve been writing about paint products for a long time, but it always seems to be the topcoats that get all the attention. I seem to constantly write about “the best water-based gloss”, or the “flattest matt emulsion”. But the most important coat of paint you apply is always your first!

Get that wrong, or use the wrong product, then your whole system fails. For that reason, I thought it would be useful if I sat down and wrote a quick guide on undercoat and paint primer. I’ll go through each type of primer or undercoat, explain what it’s for, then recommend a product for each. I hope it helps.

Knotting Solution

As the name suggests, knotting solution is used to prime knots in bare timber and prevent sap bleed. You can use it before or after your first coat of acrylic primer.

Acrylic Primer Undercoat

You’d use acrylic primer undercoat to prime bare timber before applying an undercoat or finish-product. It seals the timber and locks any fibres.

Adhesion Primers

Nowadays there are hundreds of adhesion primers on the market. Think of it as a ‘problem solving paint’. You use it on difficult surfaces like varnished wood, plastic, ceramic tiles, and plenty more. It isn’t a ‘one product fixes all’ type of paint. There are several different adhesion primers, all good for different tasks. I’ll name a couple of the industry leaders and explain what they’re used for.

Bullseye 123 Plus is a product all Decorators use.

It can be used inside or out, and will prime glossy surfaces, ceramic tiles, non-ferrous metal (such as aluminium, stainless steel and galvanised metal), vinyl, hardboard, melamine, varnish, plastic, GRP, UPVC, wood, masonry and more.

I personally use it a lot to prime ceramic tiles and uPVC doors and frames. I know other Decorators who use it as an undercoat over glossy surfaces, or to block stains, but I think there are better paint products available for that. You’ll find the application nice and easy, the opacity good, and the overall adhesion fantastic. This is a great primer.

Zinsser Cover Stain is another great adhesion primer. I use this for painting over old varnish, covering water-stains, priming glossy surfaces, bare wood, and loads more. It also makes a good undercoat for a lot of water-based paints. I used it recently to prime kitchen units before painting them with Johnstone’s Aqua Guard, and it was fantastic.

As the name suggests, this primer can be used to block stains such as nicotine, smoke and watermarks. It’s oil-based, so use a little bit of white spirit to dilute it or wash your brushes after you finish.

Water-based undercoat

Water-based undercoat is used on pre-primed, or previously painted surfaces. You can use it as a substitute for oil-based undercoat to speed things up, or as a base coat for water-based topcoats.

A lot of water-based satinwood or gloss products specify their own undercoat, and you should always follow the manufacturer’s instructions.

However, when no particular type of undercoat is specified, or you’re using a “self-undercoating” product, but need some extra opacity, I recommend Johnstone’s Aqua Undercoat. This is a hybrid paint with great adhesion and opacity, which is easy to use. I think it’s an awesome undercoat, particularly in white, but also tinted colours.

Oil-Based Undercoat

This is also used for undercoating previously painted, or pre-primed surfaces. All oil-based undercoats do the same job, but the best I’ve used is Crown Trade. The opacity and ease of use are unreal. You can dilute it heavily and it will still cover.

It easily does everything you need it to, and it does it better than alternative products.

It seems to dry quickly too. Even though the recommended re-coat time is 16-hours, I find it’s dry way before then. It’s easy to avoid brush and roller marks with this undercoat too, meaning it creates the perfect flat base for your finishing coats.

Bare Plaster Primer

Bare plaster can be primed using various products. A lot of Decorators still use contract matt, which is something I disagree with for many reasons. The best advice I can give you is to stick with the manufacturers’ instructions on your finishing product. Some companies, such as Farrow and Ball and Tikkurila have specialist primers for their emulsion paints. Other emulsion paints are self-priming on bare plaster, you just need to dilute it for your first coat.

If you want a universal primer for walls that will cope with any topcoat, then I’d suggest going for Tikkurila Optiva Primer.

Final Thoughts

As mentioned in the opening paragraph, your first coat of paint is the most important. Skip your primer or your undercoat and you will have issues with opacity and adhesion, so don’t cut corners. Even pre-primed or previously painted surfaces generally need some sort of undercoat.

If in doubt, always consult the manufacturer’s instructions. If you’re still in doubt, choose one of the products on this blog. They won’t let you down.

A Guide to Undercoat and Paint Primer – by Mike Gregory