What's the difference between a base layer and a rash vest?

The differences between a rash vest and a base layer, when you should be wearing them and why.

Peak PS Thermal Rash Vest Long Super Warm Quick Drying Base Layer

Base Layers vs Rash Guard (Eng.) – able2sportswear

Sillitor Rash Vest Mens Short Sleeve Sun Protection Swimming Shirts for Men Compression Base Layer Mens Rash Guard for Surf Diving Wetsuit Cycling,Quick Dry High Wicking Breathable Black Blue S :

SPINE-TECH BASELAYER LS (MEN)

Compression MMA Base Layer Martial Arts Body Suit BJJ Bear Rash Guard Top Bottom

New Printed Rash Guard Compression Shirt for Men - USA Made Base Layer & Swim Shirt - China Rashguards and Swimsuit price

How To Choose a Rash Guard –

Winter Cycling Base Layer (with Collar) - for Pro Cycling

Men's Intraknit™ Active Base Layer Long Sleeve

DRY - BASELAYER

Zerofit Glenn Irwin Move Baselayer – Zerofit UK