What do you do if you have messed up a decal and have no spare? I have just started to get back into model building, but this is something that worries me. I always worry about messing the decals up and ruining all of the hard work up that point. Are there places where you can buy just replacement decals?
Not replacements, necessarily, but yes, there are companies that make decal sets. The most common subjects are military-aircraft, armor, and ships, but there is a wide variety of aftermarket decal sets available. You can also find a lot of decal sets on the secondary market, eg, on eBay, or at shows, or through a club, if you have one in your area, or through forums like this.
Will the model company sell or send replacements?
Like Momo says, it depends on the company. It never hurts to contact them to ask. Same goes for replacement parts.
Any tips or tricks to get more comfortable with waterslide decals?
I trim the decal as close to the colored part as possible, with a pair of surgical scissors or an X-Acto knife. I leave just enough of the backing paper to have something to hold with a pair of tweezers.
Generally, I use a small bowl of hot water, which I nuke for a minute or two in the microwave.
I usually hold the decal with a pair of tweezers and let it soak, till it starts to lift off the paper backing, then remove it from the water. I apply it to the model, sometimes sliding it off with a finger, sometimes with a paint brush. I also use a paint brush to help maneuver the decal till I have it where I want it. I can use the brush to pick up a little more water and apply it, till I have the decal in place. The brush is useful, too, if a decal curls or folds over on itself (it happens). Then I use a piece of paper towel to wick away the water.
When and how should I use Micro Set and Micro Sol? Are they really worth it?
Yes, it can really help to use prep and setting solutions, though a setting solution is not always necessary. Some brands of decals are more finicky than others. Some are thicker, for example, and benefit from setting solutions, which dissolve the decal, allowing it to snuggle down closely on surface detail as the solution and decal dry.
A setting solution is basically a weak acid solution. Back in the day, some modelers used white vinegar-weak acetic acid. Micro Set softens the decal to get it to settle down. Micro Sol is "hotter" and dissolves it to do the same thing. Solvaset is another product, and I think it's "hotter" still than Micro Sol. I use it on the thick decals that come with Wave's Maschinen Krieger kits, for example.
Another trick I use is to apply a clear gloss coat before applying the decals. This creates a smooth surface and helps to eliminate trapping air in tiny surface imperfections, which cause silvering with the clear parts of decals. I use Future (aka Kleer) to do this. I have also used Future over a decal to provide an additional seal, or to repair a decal that lifted long after it had dried. I hit the model with a matte varnish when everything is done.
I hope that helps!