primer, undercoat, primer & undercoat

[Stephen]

Hello,

Is combined "primer and undercoat" as good as separate primer and undercoat? Surely one obvious advantage of using separate tins is that you get two coats?

Is there a big difference between primer and undercoat? I've just put primer on bare wood and found I had no undercoat so I put a second coat of primer on and it looks good to me. Have I done something wrong? Should I have gone to buy undercoat?

Thanks, Stephen.

[Peter Johnson]

Paint manufacture is changing to comply with EU requirements but it used to be that most undercoat was oil based and took time to dry. Primer is water based and dries quickly. Pimer/undercoat is water based and is usually used by contractors working on new timber - starting in the morning, it is dry enough for the gloss to go on the same day.

If you are using white or a light colour for the gloss you should be OK. For some colours the colour of the undercoat has an effect on the final colour of the gloss.

[Andrew Thelwell]

I've used combined primer and undercoat (B&Q stuff) on both bare wood and over old gloss and always found it to be spot on.

Always been painting in light colours (white usually) so can't comment on the colour thing.

[Stephen]

I used two coats of primer because I had run out of undercoat, and yes it was for beneath white gloss.

I had to go past a shed today so I popped in to get some undercoat for next time. The u/coat is half the price of primer, so I think that's one good reason to use a coat of each rather than two coats of primer. What about the other way round? Could you use undercoat to prime and use two coats of that?

The tins of combined primer and u/coat said you needed two coats which IMHO defeats the object of having a combined product. If you are going to apply two coats, you may as well use two tins!

I think u/coat tends to be thicker, whereas primer seems quite "watery" so I wonder whether this is another advantage of doing it properly that the undercoat will cover everything underneath.

Has anyone used Toolstation undercoats and gloss paints? Are they any good?

Thanks.

[The Natural Philosopher]

The function of primer is to seal and fill the raw surface.

The function of undercoat is to build up color.

If you have a tricky surface using separate things works better.