Firstly, if you're painting something really smooth (like these aluminium radiator valances), you're going to have to rough the surface up a bit. I used a fairly fine sanding block which just took the shine off the metal without leaving any deep scratches. Once that's done you need to degrease using POR15's own degreaser. This is called 'Marine Clean' and is a vaguely greenish liquid that you dilute with water and coat the part with. This normally comes in a screw top bottle but I have prediluted mine and decanted it into an old spray bottle. After leaving to soak for a while, wash off the part with plain water.
The next stage of the process requires the use of 'Metal Ready'. This is another POR15 product and is basically a blue liquid that seems to etch the metal slightly. I guess that this contains phosphoric acid but I don't know for sure! You need to keep the surface of the part wet for a minimum of 15 minutes. Again, I have decanted mine into a spray bottle to make this easier. After 15 minutes or so you need to wash the piece again to clean off any residue.
The etched surface is slightly grey and sometimes a bit 'powdery'. This is OK! You are now ready to move on to the next stage...
You now need to be quite organised to avoid making a mess. I'd advise you to use plenty of newspaper over your workbench as it's going to be near impossible to clean up if it all goes wrong when you're pouring it! Get your brushes ready and also have some cling film and a jam jar ready. It's also wise to wear gloves because this stuff is pretty difficult to get off!
So, what you need to do is to open the tin of POR15 very carefully and either pour it or use a coffee scoop to transfer it into the jam jar. It is VERY important that you cover the tin with the cling film before you put the lid back on. If you don't you'll have to use water pump pliers to rip the lid off - trust me on this! Now you're ready to paint.
I strongly recommend using a foam brush. The POR15 starter kit comes with one or you can buy a pack of 20 for £5 off eBay. This will help minimise brushstrokes and will look neater. It goes on pretty easily. Don't stress too much if it looks a bit 'brushstrokey' as it seems to level really nicely after about 15-20 minutes. If you have paint left over in the jam jar you can seal it with cling film and use it later. Stored like this it seems be usable for a few days before it drys out.
And there we go! It should be touch dry in around an hour. Unfortunately one of the disadvantages of POR15 is that it's not UV resistant. Apparently it discolours and turns brown with prolonged exposure to sunlight but won't actually fail. In order to prevent discolouration you can use another POR15 product which is resistant to UV, such as 'Blackcote', which is what I have used here.
And that's it! You're done! One of the common misconceptions about POR15 is that you can't get it off anything. They actually do sell a thinner that you can use to clean brushes etc but it's quite pricey at around £20. That seems pretty expensive when you consider that the starter packs cost around £15 and contain 'Marine Clean'. 'Metal Ready', paint and two brushes! When I get it on my hands I find it comes off easily enough if I wash them straight away. If it dries then it takes a few days to wear off!