If you have to inject it depends on how early in the season. I they are soft, then yes you should inject the solution. I've done a couple of spike elk where the velvet was firm and not spongy. In both cases I just brushed on the tanning solution. Making sure that I saturated the velvet. Then I wrapped the spikes in plastic wrap and let it set for a couple of days (refrigerated). After a couple of days I took off the wrap and let them air dry.