Painting plasterboard

[Inderpland]

So I started scraping off loose paint on the plasterboard roof outside my house and got some Jotun interior/exterior white paint. I just applied it to some of the naked areas and as you can see in a matter of minutes the paint looks like this.

So I obviously did a couple of things wrong. Any tips would be most helpful.

Edited June 8, 2020 by Inderpland

[Mister T]

Clean and primer first.

[sometimewoodworker]

If it’s plasterboard outside it’s the wrong material. 
 

it looks as if the paper layer is swelling so use clear or white shellac to stop that before you prime.

[GreasyFingers]

You may nave a water problem on the plasterboard.

[JimShortz]

It looks to me like the paint you have used has reacted badly with whatever there before. I think you need to buy a Wall Sealer (Google it) of some sort, ahead of the primer and then top coat...  Even then I would be disinclined to use that same paint on that ceiling - clearly not a match made in heaven!
It may be a bit trial and error with which product works - test a small area first. Good luck - super annoying when this happens!

[Inderpland]

Just had a look and it's not plasterboard but some kind of Huntonit or chipboard or something similar. Guess I'll buy a primer and full coat before final coat with paint.

Another question; can I use a roller for applying varnish? I have large roof surfaces covered with narrow wooden boards and it would take forever to use a standard paint brush.

Edited June 9, 2020 by Inderpland

[210]

Hi.

I just went to see my sealer old can. Delta contact primer - it says (there's a cheaper one but do no buy it!!! You may go for more expensive - sorry I do not remmember brands but there are 3 products in the market only). It's CLEAR stinky and a bit sticky substance that goes into walls, plaster, plaster board. It's way better than any primer and actually you don't need primer. I even used it before painting PVC pipes with wall paints.

The bad is that it really stinks for a week and is more expensive than primers but definitelly worth the money.

I do not know what Huntonit is and if u need sealer for it but as I got into answering your post- there we go. :)

[petermik]

On 6/8/2020 at 9:34 AM, sometimewoodworker said:

If it’s plasterboard outside it’s the wrong material. 
 

it looks as if the paper layer is swelling so use clear or white shellac to stop that before you prime.

What exactly is shellac...and where can I get it....I have similar issues,been into various DIY places here looking for an oil based primer (methinks that will do the same job) only to be given blank expressions

[sometimewoodworker]

34 minutes ago, petermik said:

What exactly is shellac...and where can I get it....I have similar issues,been into various DIY places here looking for an oil based primer (methinks that will do the same job) only to be given blank expressions

Shellac is produced from the secretion of the lac betel it the most widely used coating of everything from furniture to food products. FWIW you probably eat it quite often specially if you eat chocolates.
 

It’s enormous advantage is that it can go over every other finish and every other finish will go over it.
 

All other primers have restrictions about what they can be used on and what can be used over them.

 

its available in every hardware store.

 

this is one colour of one brand together with the colour chart

Edited June 9, 2020 by sometimewoodworker

[mrloy99]

On 6/9/2020 at 1:10 PM, petermik said:

What exactly is shellac...and where can I get it....I have similar issues,been into various DIY places here looking for an oil based primer (methinks that will do the same job) only to be given blank expressions

Aka varnish 

[sometimewoodworker]

1 hour ago, mrloy99 said:

Aka varnish 

 

1 hour ago, mrloy99 said:

On 6/9/2020 at 1:10 PM, petermik said:

What exactly is shellac...and where can I get it....I have similar issues,been into various DIY places here looking for an oil based primer (methinks that will do the same job) only to be given blank expressions

Aka varnish 

Shellac is not varnish and varnish is not shellac.