Paint over paint

Hi All,
I have 4 ltrs of Wallboard sealer and undercoat which i initailly got for undercoating new plasterboard.

Could I use this as an undercoat on an already painted surface which is a low sheen paint,the walls to be painted need cracks repaired and all walls will be sanded,though not sanded back to the plaster board itself just the paint surface.

Also from a previous room I have paint which is low sheen,this paint is the same colour as what this room I am working on now is to be painted.

Could I use the low sheen as a first coat or untill its all used up and then finish the last coat/s with the flat paint of the same colour over the low sheen?

I am just trying to save a few dollars on undercoat,paint etc and at the same time use this paint up, else it will sit for ever in the shed going to waste.

Thank you

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