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Welcome to the Bunnings Workshop community @Geoff6. It's wonderful to have you join us, and many thanks for your question about painting skirting.
It really depends on how thick the existing paint layer is. Since the enamel is chipping off the varnish, it is almost certainly because the varnish underneath was still glossy, and the paint had nothing to grip to. Your goal is to remove the loose coating, take the surface back to a sound base, and make sure the varnish is completely de-glossed so the new system can bond.
If the old paint is fairly thin, you can usually sand straight through it and into the varnish without too much trouble. If it is thick and gummy, scraping or heat-gunning it off first is often faster, then you can sand the remaining varnish. You do not have to remove every trace of varnish, but you do need to remove all gloss and leave the surface evenly scuffed. Once you are back to a matte, well-keyed surface, a high-adhesion primer is the right choice.
Dulux Maximum Strength Adhesion is made for exactly this situation and will give you the best chance of long-term adhesion if any varnish remains. Apply that over your prepared surface, let it cure properly, then follow with your Dulux Super Enamel topcoat. You do not need Sealer Binder and 1Step on top of a dedicated adhesion primer, so your process becomes simpler and more reliable.
Please let me know if you have any questions.
Mitchell
