If you can isolate any galvanized or alloy items then yes. Paint before fitting.
I come from a long line of old school painters. Im no expert in spray painting but had the honor of working with an ex coach painter for many years. The techniques he taught me have very little relevance to anything I have read on painting on this forum as most of it has been cobbled together by mechanics and spray painters. Im sure they achieve excellent results but the old ways seem to make more sense to me when using brushes. They have sound reasoning and purpose rather than guesswork and modern conformity. In the day, a coachpainter did a 7 year apprenticeship rather than the year or two a sprayer endures so I can only surmise that spraying is the way to go for ease.
For showroom finishes it was normal to apply upto 12 coats of paint by hand in various formulas and ratios, adding oils and spirits to the basic paint as the job progressed. I suppose thats why it took 7 years to perfect with the countless variations an individual job could dictate.
I have posted before on the subject and normally been ridiculed but the hand finishes I have achieved over the years speak for themselves and Im too old and stupid to change now.
Things to consider are.............
Brush on paint takes a long time to dry. Usually overnight between coats. Often longer to harden when flatting out with wet n dry between coats.
A tin of synthetic enamel is not ready for applying straight from the tin. It needs adjusting to the immediate circumstances.
Synthetic enamel is very nearly the same as modern oil based gloss paint (dulux type) so treat the same. I have actually had better and longer lasting finishes in gloss paint.
Use a good quality natural bristle brush but never a new one. A brush needs to be worked in before it can be considered for fine work. A house painter would never put a brush straight into oil paint for a skirting but will use it for emulsion for several months first so why would you for a bonnet.
Use a roller but always lay the work off with a semi dry brush from top to bottom. Vertical lines hide better with light from above. If you want orange peel finishes then use underseal and make a complete cock of it.
Use two brushes when applying. One for laying the paint on and a semi dry one for laying off, this will pick up any heavy spots round fittings that risk running.
Work fast and avoid going back into paint keeping a wet edge at all times. Cock ups cant be brushed out but must be sanded out when fully dry.
You will not get a good finish with two or three coats. You would have to apply it far too thick and heavy for this. Better to thin down and repeat.
One of the best finishes I have seen for a quick hand job (oo er) was using domestic undercoat and synthetic enamel mixed.
Pure undercoat followed by 50/50 undercoat and enamel followed by pure enamel. I cant remember how many coats it had or how many combinations (it was 30 years ago) but you get the idea. It will depend on many things how you progress with this technique you would have to use your own judgement at each stage. Each coat was thinned down with white spirit to that of thin cream and applied lightly.
The advantages for the DIYer with this is............ The oil in domestic undercoat slows surface drying time in enamel giving you precious working time. The solids in undercoat keeps the finish slightly softer so rubbing down between coats becomes easier. Undercoat is far less likely to react with original finishes. A trip to Wilko will cost considerably less than a kit of specialist car enamels, primers and thinners. HMguest (Manchester) that supply Paddocks with enamel are only round the corner.
