MDF extra smooth finish/Cracking!!

[dovey]

Hi there i have been achieving extremely smooth finishes using this method in order;

cut wood to size,
glue together to make box,
fill with 2 part filler,
sand with 60, then 320,
paint on sanding sealer, sand with 600,
paint second coat sanding sealer,
sand smooth 600

(making sure each layer is completely dry before each layer).

apply car spray paint primer (UNIPART) 3 layers, (dust free, leaving to dry each layer)
then a satin finish spray UNIPART 3 layers,
sanding after the final coat with 1200 grade wetdry with water.

i noticed someone recommending using turpentine instead of water for wet dry sanding, is this a good idea?

i have achieved a babys bum finish.
also the finish looks almost unrecognizable as wood, almost looks like metal.

the only problem, BIG problem, is that after a few weeks little cracks start to appear and form into bigger uglier looking ones!

please anyone!!

what should i do to prevent this?

and is it possible to achieve such a finish that is less time consuming and less fragile?

 

[big-all]

has the box been kept fully dry
you should never touch mdf with water
even after painting mdf should be kept away from moisture

 

[dovey]

yes! i was using a wet sand with water, should i use turps??

also i've been using UNIPART spray paint as a primer which takes many coats with a 3 coat satin black unipart finish.
do you think it would be more economical to use a thicker primer to sand down then finish with the spray?

i also realise it is important to keep the two compatible to prevent any chemical reactions, could you recommend a compatible primer and finisher? even better, compatible wood filler, sand sealer, primer, and finisher!!

sorry, i realise this may well be a tall order.

 

[foxhole]

Yep, moisture loss has caused cracking and car paint is not ideal for mdf even after priming it can still soak thru thinning the top surface.
Mdf primer is a better choice, much thicker as it needs to completely seal the surface.Whats in the BOX?

 

[active85]

Last comment is completely correct, proper MDF sealant, primer and paint. NO WATER based products allowed anywhere near the MDF. If its a high humidity day, let the MDF drie as even humidity can effect it.
It you want to get a perfect finish, hire a HVLP spray unit to put your finish coats on with. Oh, and YES, white spiril to be usedwith the wet/dry sanding but ONLY when the MDF is 100% sealed.