So it seems that every time I use epoxy I get a splitting headache that continues on for a day or two after. I use a 3m half mask respirator and the area is pretty well ventilated. Am I just sensitive or is this respirator not good enough? Or will just the smell alone cause this because I did go in the shop last night and you could still kinda smell it.

Epoxy is not as dangerous as an isocyanate hardener but you should still wear a mask with filters rated for organic compounds or use a fresh air supplied system.

With the right filters you should not be able to smell it with your mask on. Are your filters new? They must be changed regularly - see the documentation that came with yours to see what exposure level they can take, usually expressed in hours of use. Once opened they should also be stored in an airtight container when not in use.

I put the filters on less than 6 months ago and have used the masks 3-4 times. Masks is stored in a ziploc bag when not being used.

I will buy new filters regardless because last night was rough. Didn’t really sleep. Headache is gone for now though

Also I did spray surfacer after epoxy but I’ve never noticed any issues after surfacer or too costs. Only epoxy.

As Chris pointed out, you need the organic cartridges. Even then they are limited in what they can handle so if your ventilation is poor the cartridges will become ineffective very quickly. In a booth with good air flow they only last about 8 hours of exposure before going bad.

When the cartridges are expired you will loose protection from Isocyanates but still won't "smell" anything.

Epoxy overspray/dust can fill the pre-filters on the mask very quickly and reduce the amount of oxygen you are getting which may cause the headaches.

Wow I didn’t know they needed to be replaced so frequently. I’ll try buying new filters and making sure they are correct for the application.

Another factor that is generally not taken into consideration when using any type of carbon filter respirator is the shooting environment. There is an enormous difference between a pro booth and the shooting environments I've seen in "homemade" booths. Testing done years ago by the US Wood Forestry Labs for wood refinishing showed that poor air flow could allow for carts to be fully loaded with less than 8 hours use. Epoxy itself is a great sealer. My guess is it may have partially sealed your prefilters. If so, as you breathed you were pulling some air directly from the face surround seals. Trust me it happens. You wouldn't even know...... And yes, you can be "sensitized" to the chemistry of epoxies.

Good point about spray environment. I’ll see what happens with new filters next time.

I've found myself spraying thinner through my guns and cleaning without the respirator on. Then I realized how dumb that is. Might be something to think about. I try to wear it from the moment I open the paint can.