Blocking primer

[TROOPER88]

Hello
I primed my MK1 Golf project shell last week with 3 coats of 2k filler primer.

I have just started the block sanding and hoped that I could use 400 then 800 grit before paint.
To be honest using the 400 would take too long as the primer surface is coarse.

Is it ok to use 240, then 400 and finally 800?

I am guessing the 240 scratches will be removed by the 400 and so forth.

Thanks

[Squiggs]

You're fine to start gently with 240 if things are that bad. But you might want to go to (or start with) 320 before moving onto 400. Finishing with 800 is a bit extreme as paint needs something to grip onto and most paints will cover 400. It might be worth considering finishing with 500 but really it comes down to combination of what paint you're using, your application technique, experience and personal preference.

[V8covin]

Blocking a complete car will take time,there's no shortcuts if you want to do it right.
I presume you're doing it wet ? If you go with 240's there's a high chance of sanding thru which will put you back to square one.
Spray some guide coat over the shell or use the powder equivalent,flat with 400's you only need to finish with 600's if you're spraying base/clear

[TROOPER88]

Thanks guys.

Well I have just blocked the outer side (visible side) of the roof, bonnet, wings and doors using 240. It is all lovely and smooth and yes I used loads of water with a drop of washing up liquid.

On a couple of edges it has gone through but I can easily go over these with a standard aersol primer.

Next will be doing the same without the block on the engine bay and visible interior sections, underside of bonnet etc.

I will then repeat the process with what you guys suggest 4-600 grit.

Sound ok?

Then it will be on with 2-3 coats of 2k White Solid colour.

Many thanks

Paul

Edited by TROOPER88 on Tuesday 8th January 14:41

[Kidders]

Depends how thick the primer really is, and if your trying to level a panel with previous repair work. I normally use 400 dry on a block but crucially here, I see no mention of guide coat, this can be as simple as a light mist of matt black aerosol, once you have flatted the first time you can see you low points, guide coat again to level, if your happy then the finish before paint will be dependent on the colour and type of paint your using. Generally 500 dry on a D/A is good for most systems, any edges you cant get to, I use 1000 wet to finish.

Without the guide coat your doing it blind and could easily miss imperfections like deeper sanding marks, pinholes, and filler work not quite good enough underneath. To some extent you can flat the primer to correct these but really it should be to a good standard before primer.

[TROOPER88]

Kidders said:

Depends how thick the primer really is, and if your trying to level a panel with previous repair work. I normally use 400 dry on a block but crucially here, I see no mention of guide coat, this can be as simple as a light mist of matt black aerosol, once you have flatted the first time you can see you low points, guide coat again to level, if your happy then the finish before paint will be dependent on the colour and type of paint your using. Generally 500 dry on a D/A is good for most systems, any edges you cant get to, I use 1000 wet to finish.

Without the guide coat your doing it blind and could easily miss imperfections like deeper sanding marks, pinholes, and filler work not quite good enough underneath. To some extent you can flat the primer to correct these but really it should be to a good standard before primer.

Thanks for your input.

The paint is 2k solid white.
The shell had been painted (a not very nice blue) a couple of years ago so there were only a couple of small filler repairs to note. The rest of the shell was very clean and straight.

As said I have just about blocked the exterior with 240 wet on a block.
I did not spray a guide coat as I knew the shell was straight but in hindsight it sounds like I should have.

I will give it a light dusting of matt black now before I sand again with 400-600 before paint.

Thanks again

[V8covin]

You should use a guide coat all the time,even on filler especially if it's a large area.
If you've sanded thru in places with 240's you're going to sand thru some more when you go to 400's,the primer is getting thinner every pass