The reason this happens is that when you seal the model, atomized micro-droplets of water will become trapped in the finish. If you try to seal a model in high humidity, you are an asshole for tempting fate, just as I have been so many times, living in the South. You should know better. But, alas, I have compiled the wisdom of my own lessons learned as well as those on the internet and have decided to provide you some sure-fire methods to sort that shit right out, restoring your model to its initial magnificence:
1. Rubbing the model with a cotton wad soaked in olive oil can remove the frost. This is particularly effective for scenery with flat surfaces. Not so much if you can't get into the nooks and crannies (TM). The oils drive out the surface water droplets, and then when you wash the model in soapy water, the oils wash away and your model should be restored. This doesn't always work, but it's a simple first line of attack.
2. Future Acrylic Floor Polish is the most underrated and unknown sealer on the planet. The shit is magical. If you brush on a coat over the model, or even airbrush it (thinned), it will put on a super-durable sealant, and then when winter comes and there's no humidity, you can spray dullkote over it to regain that matte finish. Now, if you DON'T have this in your stock already, it's clear you're an amateur, because as I noted, this shit is magical. Use 50/50 Future and Water and then add small amounts of color (read: paint) to create incredible washes and inks that seal as well as pigment your models. Incredible shit.
3. Use a hair dryer or heat lamp! You can SOMETIMES remove frost by using a hair dryer, heat lamp, or household small room heater. The frost is actually water micro-droplets that get trapped in the sealant, which causes that hazy effect. If you use a heat source and get the model up to high temperatures, you can steam off the water vapor and restore the model. I shouldn't have to say this, but I will: Don't do this on plastics unless you want it to turn into a pile of melted plastic. Also be aware that some paints have a very low smoke point and you can literally burn black marks onto your models if you're not careful.
4. Toss that bad boy in the oven, set for 225F, in order to achieve the same effect as using a hair dryer. The boiling point of water is 212F/100C and thus you can put a model in the oven for 10-15 minutes (preheat the oven!!) and drive off the water. The downside to this method is that plastic bases, flocking, and other wee bits can melt. This should be a last resort on a competition-quality model, and remove the base when you do it or you're probably going to melt it anyhow. Again, ONLY METAL MODELS, and be aware that you run the risk of blackening or charring the model's paint job.
5. SEAL THAT BITCH AGAIN! You can spray right over the model once it's dried for 24 hours, and it completely removes the frost effect. This is the obvious one, but people get scared that they'll seal on the frost and irrevocably fuck their shit up. Nope, just reseal and you're good. I shouldn't have to say this, but again, I will: Don't spray it again in humid weather or you'll just repeat the results and add more layers of frost. Either spray it outdoors and then bring it into a humidity controlled environment, or just wait for a good spray day. You can wait years to respray, too; it's not as if time somehow makes any difference in the impingement of water vapor under the sealant. Patience is absolutely your friend when it comes to the hobby of miniatures painting.
For me, I think the BEST POSSIBLE METHOD of sealing a model is to use brush on sealant, because you never over-spray, and you can control the thickness. Brush on sealants are also always a better protective coating because of the viscosity and the thixotropic nature of the varnishes on the market. An urban legend exists that says that matte sealants are inherently weaker than gloss, but allow me to dispel that myth in one word: BOLLOCKS. The best way to seal a model is with a brushed-on gloss coat of Liquitex Gloss Acrylic Sealant before getting sprayed with Dullcote or Krylon Matte.
Brushed-on coats are thicker and provide a better seal, and on top of that, the gloss finish brings out some of the colors better, especially blacks and metallics. They're also easier to spot if you miss an area, especially under bright light. The matte spray-coat over the top of that adds an extra layer of protection as well as removing the shine and making the model look more realistic. That, and the multiple layering of sealants cause rough handling and dude-drops to bounce off my models' paint like bullets off of Superman's balls.
Hope this helps you out, I know I struggled with this damned Kentucky humidity for YEARS and finally realized that you can't fight nature...it's simply better to wait until night when the humidity is lower.