From the sounds of it the flux you are using for the Tinning application is not strong enough to give the nozzle a deep Solderable finish hence why you are having to re tin so frequently.

Having looked into these special fluxes required to Tin the nozzles it took a long time to get the right one and they are strong based fluxes so a normal Assembly flux would not be suited for this application. Once you try the right flux it will put a good deep coating back on the nozzle

The material to use for wetted nozzles is cold rolled steel. The reason for this is so they can be tinned. If you use stainless, or titanium, or some other steel they will not tin correctly and your soldering results will be very bad.

Because of this, they are not protected from Pb free alloys which is why they will not last a long time.

If they have gone from once a day tinning to once an hour then chances are the nozzles are wearing out (walls getting thinner), and/or you have changed the type of flux being used to tin, and/or they are not being cleaned properly.

Never use an alcohol based flux to clean them because the alcohol will burn off to fast and you will see a lot of black burn marks and residue on the nozzle. Then when cleaning them you will scrub them too hard to get the burn marks (residue) off and this will contribute to the thinning walls and the wearing out of the nozzle. You will also end up tinning them a lot more often.

There are several types of fluxes that will do a good job for tinning. You can go to your local Lowes store and get some Lenox Water Soluble flux and this works really well. It's not a paste and not a liquid. It’s like a hand cream. Remember, the idea is to Gently brush the oxidation off. You do not need a lot of pressure to clean these.

Some selective machines on the market allow you to use both wetted and non-wetted nozzles, and some machines even allow use of them both at the same time. There are advantages and disadvantages of both. The wetted nozzles allow you to get much closer to SMD, and you can usually go down to about a 1.8mm nozzle.

However, there is more maintenance on these, and they are not protected for Pb free alloys. On non wetted nozzles you get a lot more energy allowing you to solder a little bit faster, and good results are generally easier to achieve. There is virtually no maintenance on non wetted nozzles and they will typically last more than 5 years even while soldering with Pb free alloys.

The only disadvantage of the non wetted nozzle is you need a little more keep out area (about 3mm) vs. 1mm with a non wetted nozzle.