Hi @JayBee1,
Welcome to the Bunnings Workshop community, it is fantastic to have you with us.
If the old oil-based enamel is still well bonded to the timber and not peeling, flaking, or lifting, it can usually be prepared and repainted successfully without stripping. Paint stripping is only necessary if the coating is badly deteriorated, such as areas where the paint is peeling, bubbling, heavily cracked, or excessively thick and brittle.
Sanding and then priming should be sufficient in this situation to ensure the new paint sticks to the old. The fact that water-based paint was previously applied over it without major issues is a good sign that the surface beneath is relatively stable.
For sanding, start by using 120 Grit Painted Surface Sandpaper to remove the gloss from the enamel and smooth out minor imperfections. Once this is done, follow with 180 Grit Painted Surface Sandpaper to create an even, lightly keyed surface to help coatings bond to the surface.
After sanding, use sugar soap and a microfibre cloth to thoroughly clean the surfaces to remove all dust and residues. Rinse with clean water and allow the timber to dry completely before proceeding.
I'd then suggest you apply Dulux 1L PRECISION White Maximum Strength Adhesion Prime, before applying two coats of your chosen paint. This primer will bond well to the old oil-based enamel and will ensure the new enamel paint sticks.
Let me know what you think, and if you have further questions, please don't hesitate to ask.
Jacob
