In my experience, there are two camps of people who invest in purpose-made clothing for the Camino.
The first group, including me, takes in those who prefer the synthetic "technical" fibers, for their light weight, wicking properties, insulation both against cold and heat, resistance to stains, fast drying, and designed-in sun protection.
I personally wear Underarmour short sleeve polo shirts. The flip-up collar helps protect my neck from the sun on hot days and the cold on, well cold and or rainy or snowy days. The collar is also more "dressy" when in a town or city in the evening. However, after grilling my arms on the Meseta this year, several pilgrims from OZ convinced me to use light colored, long sleeve polos next year.
The second "camp" is comprised of folks who swear by natural merino wool. They correctly point out that most of the natural properties of merino wool are the same as the synthetic fibers. I have tried both, and I respect both positions. From my experiences I developed a few principles and observations that you might find useful:
1. Natural wool, when wet, smells like - well wet wool. During May this year, I had occasion to deal with a flock of perhaps 200 or more sheep being led down a country road on the Meseta by their shepherd, sheep dog, and sheep donkey (yes, you heard it right - a sheep donkey). After that experience with these filthy, wet, very aromatic, but still awfully cute creatures, I cannot countenance my wool sweaters. I will likely get over this trepidation with time. But, believe me when I tell you that when wet, synthetic does not smell, while natural wool does. Merino wool smells less like wet wool, but if it contains lanolin, which it should, it will smell. it can make some folks nauseous...plan accordingly.
2. Merino wool does dry fairly fast and does insulate to a degree when when wet. But in my experience, synthetics dry faster. This becomes an issue when it rains for five days straight, your albergue is not heated, and the clotheslines are all outside - in the rain. You can roll and stomp on synthetics in a micro-fiber towel and put them back on if need be, even if they are still damp. Wool does not dry quite this fast. Neither fabric should EVER go in a clothes dryer.
3. Merino wool is superior for cushioning where cushioning is paramount - PERIOD. While I wear two pair of socks, the inner is Cool Max synthetic, and the outer is Smart Wool from New Zealand. I have yet to find a synthetic that provides the same "feel" and cushioning as merino wool-blend socks.
4. Undergarments are HIGHLY personal. My experience is that on a hot sweaty day, synthetics dry faster and keep you drier than does merino wool of similar construction. As a guy with nil space between his thighs, I prefer loose-fitting, synthetic boxers.
For my money, the best brand on the market is made by Ex-Officio. But this is my opinion. They advertise, "six countries, three weeks, two pair..." or something similar. They hand wash wonderfully in any sink, wring out nearly damp, and are good to go after maybe 4 hours even in a cold, damp albergue. I have worn just washed Ex Officio boxers after the " rolled in a towel stomp." They were fully dry in 30 minutes of wear.
I did also try a pair of merino wool boxer shorts. They were hotter, thicker, and less roomy than the synthetic type. For me, the choice was simple. I could not stand the "binding" feeling I got from the merino wool boxers. A guy has gotta be "free." Anything that even MIGHT chafe is off the consideration list.
My bottom line on "bottoms" is that you need to wear what is most comfortable for you, allowing for a six-to-seven hour walking day, in all conditions, wet or dry, with sketchy ability to wash and dry overnight.
In parallel with the OP (above) who commented that: "In Afrikaans we have a saying: "Goedkoop koop is duur koop" (by buying cheap stuff, you might be ending up paying more)" I have a corollary saying that I've learned over the years: "You will remember the value of the well bought items LONG after you have forgotten the cost.
At 61 years old, I found that is true for most things one buys. Clothing, footwear, and your rucksack / pack for the Camino is NOT the place to scrimp.
Regardless of which way you go, test it first. I hope this helps.
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