Plasterboard Joints Showing up after Painting

[rod1949]

I've got this hallway leading into our bedroom, the plasterboard sheets run horizontal with one horizontal joint, flushed off nice and smooth and flat and level. Over the weekend I put the finished colour x 2 coats on all the walls (hall and bedroom). The joints in the hallway now appear with an indented look. What have I done wrong ?

The paint used is Wattyl Sealer/undercoat and 2 coats of Wattyl Id Low Sheen satin finish.

[pawnhead]

You pressed too hard on your broadknife/trowel, and you held it on an angle when you applied your first coat of base.

First coat, I usually overfill the joint, then hold the broadknife at 90 degrees to the wall when scraping off the excess. The idea is not so much to get it smooth. subsequent coats will do that. The idea is to get that first coat flat. It doesn't matter so much if there's scratches in the surface.

Of course you can get it smooth by holding the knife on an angle and pressing a bit, but then you end up with a depression like you've got.

Give the depression a bit of a sand, and hold your knife at 90 degrees for the first coat this time. It can't flex into the depression that way. :wink:

[rod@plasterbrok]

Pawnhead I dont totally agree with you. I would never recommend over filling the first coat then scraping it back level! the fist coat should be under filled an not wider than the rebate. The 2nd coat if for getting the join filled almost flat.

Then scraping back any excess, more often trowell marks etc.

Holding the braod knife on an angle is required when filling a join or else it spreads to wide. Come through the join with the broad knife at 90 degrees for the susequent runs after the mud is applied.

The idea is not to get the first coat flat!!! That is how you get over filled joins.

Rod has underfilled the joins for sure but not for the reasons you state.

Rod as pawnhead has suggested just re trowell the join with topcoat. You can easilly see the underfilled areas when it is painted. You need to use a 12" trowel or blade. When you are done sit the blade on the join and see that it is filled correctly.

If you run a level over the join after you first coat of paint any underfilled or over filled areas will show up like dogs b----. The metal of the level will leave a black mark on any high spots.

Give it another go and don't over sand it when you sand.

Many plasterers use a 2 coat system where they attempt to get the first coat flat and top coat over it. We have banned our tradesmen from using this method as it results in over filled joins more often than not.

Cheers Rod

[rod1949]

Thanks Fella's

To Rod@plaster to re-do the joint I assume I will have to remove the paint back to the plasterboard ? or will the mud (I've used Gyprock's combined base and finish mud) stick to a painted surface that has been roughed up by just sanding.

Rod 1949

[pawnhead]

Rod's the expert so he'd know best, but I'd just rough up the surface with some coarse sandpaper. I wouldn't bother trying to get all the paint off. You'd be there for ages, and clog up a lot of sandpaper. I've topped over ceiling white without sanding before and I've had no problems, but that's a flat paint. I wouldn't do it without sanding paint that's got any hint of gloss. Low sheen is wash and wear, hence it's designed so that stuff won't stick to it.