External corner mitres- I would use two pack filler. It will survive light vacuum cleaner knocks but when you sand it, you will end up sanding through the primed finish on the MDF. All of a sudden you will have areas of dark brown that will need numerous coats of "primer"/etc to hide.
Where the skirting meets the architrave- I would use a soft filler such as Toupret RedLite. It is easy to sand and wont shrink a couple of months down the line.
The only place I would use caulk is where the top of the skirting meets the walls and internal corners.
Your new architrave- that looks like a 5mm gap. If you caulk it the caulk will just shrink back and leave an indentation. In the absence of timber fillets, I would use 2 pack filler but that may be beyond your skillset. And no, I am not intentionally being condescending- it is difficult to work with without requiring careful sanding.
Once the gap is cleanly filled, you can caulk it.
I strongly advise against the powdered fillers that you mix with water- they are fine on plaster but will eventually "blow" on wood.
Where the skirting meets the architrave- I would use a soft filler such as Toupret RedLite. It is easy to sand and wont shrink a couple of months down the line.
The only place I would use caulk is where the top of the skirting meets the walls and internal corners.
Your new architrave- that looks like a 5mm gap. If you caulk it the caulk will just shrink back and leave an indentation. In the absence of timber fillets, I would use 2 pack filler but that may be beyond your skillset. And no, I am not intentionally being condescending- it is difficult to work with without requiring careful sanding.
Once the gap is cleanly filled, you can caulk it.
I strongly advise against the powdered fillers that you mix with water- they are fine on plaster but will eventually "blow" on wood.
