A Guide to Undercoat and Paint Primer

Updated Feb 14, 2024 | Posted Jul 31, 2023 | Professional insight, Product Advice | 1 comment

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.

There are a handful of primers on the market which you can use to prime the timber and knots together, but knotting solution is always the saftest option. You simply dab it over the knots and it seals everything in. Most knotting solution products are shellac-based.

knotting solution to prime wood and prevent knot bleed

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.

If you want the best, then look at Joncryl Primer Undercoat, simply because the opacity is good, which means less topcoats. It has fairly good adhesion qualities too.

Another product to look out for is Valspar Trade Knott Block. The opacity isn’t very good with this option, but it does blocks sap which would otherwise bleed through your paint finish, so no need for knotting solution.

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.

oil-based undercoat for use with gloss or satinwood

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.

This maybe overkill, but at least you will be using something that you know will work. Great opacity, easy to use, good adhesion, and a nice smooth surface.

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

Updated Feb 14, 2024 | Posted Jul 31, 2023 | 1 comment

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1 Comment

  1. linda baker

    hi,can you tell me what type of primer i need to use to paint over ceramic tiles before wallpapering over them with vinyl anaglypta thanks

    Reply

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