Second hand weight plates are fantastic value. I have 200+kgs of good quality iron weight plates, in a really nice range of sizes, and they cost me nothing (check out this blog entry). However, second hand weight plates are usually dirty and rusty, and many people are tempted to paint them to make them look sharp again.
From my experience, painting old weight plates is not the best thing to do. About five years ago I decided to paint my York weight plates. These plates were used to train clients and had become grubby, chipped and rusty. I cleaned them up, painted them with black paint (designed for iron gates) and they looked fantastic. It was quite a labour intensive process, but I was pleased with the results. Then I began to use them, and within weeks they looked awful (worse than before I painted them) because the paint chipped really easily. If you now want to remove all the paint, it's a lot of hard work involving paint strippers and/or mechanical methods. I've been here, and it was a pain in the backside.
When I purchased 200+kgs of weight plates during the COVID pandemic, I was determined to make them look smart and professional. Painting them hadn't worked for me in the past, so I did some research and ran some experiments. I concluded that the best finish for iron weights plates is their original finish, tarnished or not, but cleaned and protected.
From my experience, plain black plates come out really nicely (think of the old-fashioned York ones) because the difference in colour between the factory paint and the bare iron is almost unnoticeable. However, if you have plates that were factory painted in other colours (blue is common in older plates and I also have 4 x 10kg plates that were yellow) they don't look as good because the bare iron really stands out. If you, like I did, decided to strip these plates back to bare iron, you're in for a lot of labour.
Here's how to clean iron weights plates (and also barbells and collars):
Give them a quick wipe down to remove dirt, cobwebs etc.
Fill up a wheelbarrow (or similar large receptacle) with a solution of water and food-grade citric acid (about 40g citric acid powder per litre of water). Citric acid is easy to get on Amazon. This solution is brilliant for removing rust.
Submerge the plates in the wheelbarrow with the solution of citric acid and water. Make sure the citric acid has dissolved.
Wait for as long as you like! I left plates for 24 hours. The water will start to smell a bit funky towards the end of this.
Get a bucket of clean water, rinse the plates and dry them out with old rags. At this stage, you might be amazed at how clean the plates are. You might also be surprised at how dirty your rags become as you wipe off the rust (so have plenty of rags).
If the rust is very stubborn, you can consider a slightly more aggressive approach:
Scrub with a wire brush
Use an angle grinder with a wire brush attachment
Rub with steel wool.
By now your weight plates should look very clean, and if they're very old, they'll have a cool patina that I think looks much better than brand new plates. See image below:
The photo below gives you and idea of what these weight plates looked like before cleaning.
Here's how to protect weight plates (and barbells and collars):
Buy some WD40
Find a clean old rag
Rub WD40 all over them and leave to dry. This is a technique that has been used to keep tools rust free for donkey's years.
The weights plates will eventually start to show rust if left in the right conditions (in my case a cold and damp shed) so you will need to periodically wipe them down and reapply WD40. It's really not a big chore.
On a final note, there is one alternative solution to your weight plate problems: just don't worry about it. Weights are ultimately for lifting not staring at. I have gone to this effort because it is important, from a business perspective, for me and my kit to look smart and tidy (reasonably, most of the time...).
EDIT - Find out how these weight plates looked over a year later here.