If by coarse you mean big strong individual hairs, and you are low-porosity, rejoice, you have the most damage-resistant sort of hair on the planet. And if you are coarse, low-po, and loosely curly, we are hair twins!
I agree wholeheartedly with multi, if your hair is actually coarse, you may be using products with too much protein if you are getting frizz all the time.
If your hair is damaged from flat ironing, it is not low porosity (or not anymore!) so you may want to be aware of that.
Mine is over on the coarse side and very low-porosity, never gets split ends or damage but yes is prone to dryness and frizz if not well maintained, and will always frizz without gel for styling and enough moisture in the hair. (without gel it may act like it's going to dry without frizz, look good wet and half-dry and almost dry, but as soon as it's dry it will be a mess. without enough moisture it will not look good even with gel.)
For day-to-day use I do DevaCare low poo and knockoff fake Biolage Conditioning Balm from walgreens or sally's. Style with Re:Coil and ArcAngel, or sometimes leave-in conditioner instead of Re:Coil. For deep conditioning (weekly in winter, less often in summer) CJ Curl Rehab, Ion from sally's for protein tx (every 2-3 months or so when/if hair gets droopy)
It is good to have coarse low-porosity hair.
If your hair is really coarse, it will likely improve if you increase moisture and decrease protein, and you may always need gel for styling.
If you have harmed it by too hot/too much flat ironing it may not be low-porosity, and may actually be damaged, not coarse.
2c / C / iii (FIA)
Med-Coarse, LowPorosity, NormalElast (LCLF)
Wash: DevaCare Low Poo
Detangle & Rinseout: Walgreens or Sally's Conditioning Balm (always)
Leave-in: DevaCare OneC (budget choice is GFTN)
Style: ReCoil & DevaCare Arc Angel (budget choices are HESMU or Got2BSpikedUp)