Steps on slowing the rot.
Get the guards off and the vent/ grill off as well (plenum grill?.. back of bonnet). You may want to soak the bolts in a bit of WD40 before you try to remove them. If the gaurd hasnt been removed in its life, chances are that you are going to snap the heads of the bolts if any rust has grabed them. Work at them slowly.. no rush.
The two bolts at the bottom of the rear of the fender pannel will be the mongrals. Soak, heat, tap.. they are nearly always rusted in. Think about replacing the bolts with stainless items and a bit of Locktite. It will save the hastle next time around.
OK, get the hose out and your board shorts on and wash out the 25 years of crud in the splash and drain channels under the vent grill. Dont wash crap down the air-vent holes. Stick your head in the hole and check where things are first. Scrub out the gunks and get a bit of coarse wet and dry and rub down any rusted bits. Wash down again and let dry. Use a phosphoric based rust converter/ sealant paint. The stuff reacts with rust oxide and bonds to bare steel; it also bonds to paint and skin so take care. Give the splash and drain tray a good coating and include any areas which look like they could be rust catchers in the future (seams, welds, corners etc).
Now, the water that enters through the grill and into the splash tray is supposed to run to the outside edges of the tray, through a 2" rectangular hole and empty down the base of the guard. If you note where this rectangular hole is inside the tray cavery a few extra drainage holes can be drilled from the outside of the section into the area of the tray where the water drains. Once drilled, clean the site up and paint with rust converter. A couple of coats of very good quality metal paint on the inside and unseen parts will help keep things like rust at bay.
Last step is to keep it clean. Make it part of your habit to wash out the splsh tray, check for leaf and dirt build-up and also give the inside arches a good wash down too... And polish those stainless bolts while youre down there!

Most rust removers contain an acid called phosporic, it it commanly found in Coca-Cola !! Sprayed in a 5:1 solution with water it etchs the steel and leaves a nice matt gray colour. Phosphoric that is not coke !! You could patch that area with stainless by using a Mig welder it will happily weld the two dissimilar metals. Over the last 5 years or so all the rust repairs I've done, I've used a paint called POR 15 it is brilliant . POR is supposedly short for paint over rust. Allowing the area to drain is important but that area tends to collect crap flung from the tyres etc quite quickly unless you can devise an inner mud guard.

Stainless steel. Sorry, that was uncalled for. I'm in the same boat, RUST! I found a product in the USA you spray/brush/soak on, it's called "must for rust". It has already(in 15 min) neutralized the rust in the trunk. I will probably improve the drain too.