Re appearing crack along plasterboard break

Hello, so I was boarding a cupboard and a board broke in half, which I thought filling would be fine.
Filled and painted about 3 tines. The crack always came. Eventually I fibre taped it and easifilled over. Painted and the paint cracked along the line. Then I easifilled a larger section and the easifill didn’t crack. As soon as I painted it, the poxy line comes back but more a raised bump. This is more annoying not being able to rid of it than it being in an important place. ...... Anyway. A decorator told me to sand and paint over it with oil based undercoat..... which I did. Now I have a slight raised bump but a little less with the poxy undercoat glaring through and not disappearing after 3 coats

Constant cracking, especially after applying scrim, sounds as though there is movement in either the boards or the framework. If you push on the boards and the crack worsens or lessens, you will need to ensure the boards are re-fixed to eliminate the movement. No form of paint will hide an unstable crack for very long.
Other than that, it's difficult to say why the crack keeps reappearing.

The boards are secure. It doesn’t lessen or open wider with pressure. There are heating pipes running up behind it. Is there anything I can do if it’s a problem due to heat?

burts said:

The boards are secure. It doesn’t lessen or open wider with pressure. There are heating pipes running up behind it. Is there anything I can do if it’s a problem due to heat?

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Unfortunately, the heating pipes are likely to be the culprit and there's not an awful lot that can be done. The problem being that, when the heat is on, the timber will lose moisture causing it to contract and move. The opposite is true when the heating is off. The timber will eventually reach a point where it becomes more stable and the cracking should stop, but depending on how humid or dry the room gets, it may never stop. The only guaranteed way around it is to avoid having a joint where the heat source is, but that may not be possible for you.

misterhelpful said:

Unfortunately, the heating pipes are likely to be the culprit and there's not an awful lot that can be done. The problem being that, when the heat is on, the timber will lose moisture causing it to contract and move. The opposite is true when the heating is off. The timber will eventually reach a point where it becomes more stable and the cracking should stop, but depending on how humid or dry the room gets, it may never stop. The only guaranteed way around it is to avoid having a joint where the heat source is, but that may not be possible for you.

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It is possible, as the board was accidentally broke when it went in, so it could be replaced in a full sheet but I never for one second thought the pipes were close enough to cause so much drama being filled. As it’s a cupboard, I will just leave it.
I just hate it when you know something is there !
Thanks anyway