Depends on how often you use it,whatever you paint it with is going to get a hammering if you use it often,perhaps a better but more expensive option would be to get it galvanised at a spealist or a nutty solution get it chromed as custom car people get thier engine parts chromed.
Paint wise a £5 tin of spray etch primer and a good coat or two of hammerite is a cheap option.
The short answer is that you don't, you have it regalvanised. There will be someone advertising in your yellow pages who does it, and it isn't too expensive when you consider what a can of paint costs these days. I last did one about 20 years ago [£10-20 ?], so I can't help with current prices. I can remember that it seemed like a bargain; buy a big, ratty looking, rusty 70 lb [or so] CQR for £15, have it regalvanised for roughly as much again, and it looks like new and seems cheap.
There is often a 'minimum order, which makes it look expensive, but there is usually some way round that, like getting a few others to go in with you, or saying that you aren't in a rush,and you will wait until it's convenient for them to drop it in with another order.
My old CQR had some lead in the hollow of the toe, which was lost in the process, but can be replaced, in my case by cement.
If you do paint the anchor, this makes the regalvanising option less practical, as every scrap of paint must be removed first. I think this would even apply to a zinc based paint like Galvafroid, but if I had to paint an anchor, that is what I would use, because it looks the same as a galvanised finish. It is a soft paint, and would not last very long in use on the seabed, but it would look OK.
I still have the big CQR that I had treated, and after all this time it is about as rusty as it was when I bought it, but it no longer offends me [my standards must have slipped]. I also find that it works just as well when it is rusty!
I recently had two 50lb anchors made to a design which I saw in a magazine somewhere (PBO maybe?). The anchors cost me nothing to make but it cost £25 each to get them galvanised at a local industrial estate. Still a bargain I thought.
Yes you should paint your anchor.. Not for protecting it against rusting. .. if you are anchoring quite often, and mostly in sand or sand with shells.. the paint will only last a couple of anchorages.. mostly at the tip level.. and for rust protection, the only mean is a good hot dip galvanization process..
But, if you have the chance to anchor in crystal clear water.. you will be happy to have painted your anchor in white or better in bright yellow.. (That’s why the fluke of the SPADE anchor is painted in bright yellow..) I never leave my boat without rowing my dinghy over the presumed location of my anchor.. If I can see some yellow color, it means that the anchor is not fully buried in and then I have to take the decision to go.. or to restart my anchoring maneuver.. Easy to do.. and it will ad a little bit more of security to your anchorage..
Painted my galvanised anchor with Prima-Con (anti foul primer) then two coats gloss.
Lasts about a season, then re-paint gloss, but at least the anchor looks better on deck by the bow where it should be.
Take your anchor to a friendly steel fabricator who can put it in with the stuff he wants galvanised.
You might have to wait a bit longer but if he is a decent bloke it should not cost you much.
Yes find a firm who will put it in with their gavanizing. I had a small item done a few years ago by Hayling Trailers for a fiver: their minimum, but I could have had between 2 and 3 times as much done for the same price. Compared well with the minimum prices quoted directly by galvanizers.