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Filler on mdf?

I wanted to ask what you have found best practice for filling mdf.
I have a house that has all the skirting boards replaced with mdf 9 inch moulded skirting. Its been primed and fitted with screws so the holes are in need of filling. I am a little concerned that two pack although it will rubb back nice, can push out over time as I have seen it pop in the past. One fill has a bit of give but is not so good to finish on this kind of hole.
Any thoughts?

Hi ciaran use bonda decor fill,excellent product.

if any doubt about two pack popping out try Repair Care,

I’ve used Repair Care occasionally for interior work. It holds it’s shape well so ideal for mouldings and outside corner edges that need attention

I use Ultima plastic padding from Halfords, it has more flexability than most 2pk fillers as its designed to be used on plastics.

Anything that is water based can cause a problem with mdf as the bare mdf can absorb water and swell, pushing out the filler,

You could prime with Cover Stain prior to filling or use a filler that isn’t water based.

Has anyone noticed how awful the primer is on a lot of new mdf trim now? I had some the other day that softened with water and could be rubbed off with your finger. All well and good as a cheap flash over for manufacturers, but not to good for a long lasting paint job.

1 Like

Thank you all for your replies.
I went for two pack and I am getting the slight swelling with two pack even which i have cut back, hit with cover stain and after rubbing down again filled again to fan very subtley whats left as i cant get it to sit dead flat even after all this, lots of processses to get a good finish!

Mr Taylor, I had reasoned that cover stain into the bare hole might do the trick as well so have tried my second room with this method will be ready to rub monday so will see if it makes the difference.

Russ whats the Ultima like to rubb? I am finding all the two packs i use seem to be staying that bit too sticky regardeless of how much hardner i add…also noticed that so called white filler (ronseal) changes colour to almost yellow…the hardner might be older than the filler any thoughts?

Hi zen there’s one product that will sand to a perfect finish,that’s marine filler,i have used this repairing windows ,on boats,this wont let you down and any fool can get a perfect finish its made by the same company that makes plastic padding,sets in 10 minutes and overpaintable.

Hey J
Thanks for that. The problem though is not so much the filler, although as I mentioned I am seeing the same issues with most two part filler. It is the way mdf responds to the filler ie it swells leaving a raised surface where it needs to be flat. When rubbed back then refilled the process starts again. …mdf is the problem not the filler. As Mr Taylor points out putting Zinnsser cover stain on mdf allows one to sand back the raised particles in mdf. So hitting it with cover stain prior to filling would seem to be the logical step.

A nother MDF/filler question: A corner has been knocked off the an MDF drawer front. I was thinking Repair Care SF that says it needs no primer to bond to ‘wood’ or go the whole hog and mix up some Repair Care Dry Flex 4, pretreating with the Dry Fix 4 so that the Dry Fix penetrates the MDF and the Dry Flex then bonds to that - maybe making a more durable corner repair? An expensive option has you have to mix up much more than I need to make sure the proportions are right. What do the Kitchen Painters do when they get corners and edges on MDF that need fixing? A rapid response would be appreciated! Thanks all.

i go the route you mentioned with the full Dry Flex - expensive and sometimes difficult to gauge the exact amount - but the adhesion is so good and reliable for the long term.
it sculpts and shapes up so well for mouldings (I repaired a series of skirting board corners chewed up by resident dog!)

Repair Care do a single cartridge with both components in for small areas. Yes its even more expensive but I carry one, sometimes use on door mouldings

I don;t know if I’d use the SF for anything other than a rapid dry fine surface fill on a flat surface but open to correction on that

Whats the best filler for MDF coving?cheers

Used tx110 worked really well hope that helps someone.