When painting with Gloss paints the smoother the surface your painting the higher the sheen you can achieve, in other words you can prepare and paint a brand new steel door in exactly the same way as a timber door, and you could have done a perfect job and be really pleased with the timber door but the more smoother steel door will have a higher sheen, (due to the way light reflects) so bearing this in mind whatever you paint the smoother the surface the better.
For this example I will explain painting a factory spray primed steel door.

Tools
1x paint 2.5ltr paint kettle
1x 1” Hamilton perfection brush
1x 2” Hamilton perfection brush
1x dusting brush
1x Bucket and sponge

Materials and sundries
Trade undercoat
Trade gloss
Tack rag (a sticky cloth when wiped and dabbed over surface picks up dust and grit)
400 grade wet and dry abrasive paper
White spirit

The new spray primed steel should be perfectly flat and smooth.
Step 1
Lightly rub down primed surface to provide a key then dust off with dusting brush.
Thin down undercoat about 5% and apply to door with 2” brush and lay off the paint in one direction. Because the paint was thinned down the texture of the brush marks will be less noticeable than if used neat from the tin.

Step 2
Lightly rub down any grit (de nib) that may have got in the surface of the coat applied and dust off.
Step 3
Thin down undercoat about 5% and apply to door with 2” brush and lay off the paint in one direction. Because the paint was thinned down the texture of the brush marks will be less notice able than if used neat from tin.
Step 4
Lightly rub down any grit (de nib) that may have got in the surface of the coat applied
Step 5
Now get a bucket of water and dip in the bucket a piece of 400 grade wet and dries paper and lightly rub out the brush marks. The 400 grade being quite fine and in conjunction with the water cuts out the brush marks and polishes the surface up to a really smooth base for the gloss, when this is complete wash down with clean water and a sponge.

Step 6
Prior to glossing dust off and go over the surface with a tack rag (available from decorating merchants), now you’re ready to gloss.
Thin the gloss down slightly about 5% to allow the paint to flow better and lay off in one direction.
Step 7
Finally lightly de nib the surface, go over with a tack rag, then thin the gloss down slightly about 5% to allow the paint to flow better and lay off in one direction You should now have a gleaming shiny door as you had smooth steel to start with, then applied your undercoats and took out the texture of the brush marks with the wet and dry paper, and now applied the free flowing gloss that should be brush mark free.

If you apply the principles of getting your surface as smooth as possible to start with by stripping, filling, and feathering out imperfections, then apply your paint slightly thinned to flow better and then rub out with a fine paper the brush mark texture, then you can achieve higher quality gloss finishes to old painted surfaces of timber as well.